History of DC-3 c/n 20215 by the authoritive Air-Britain publication The Douglas DC-3 - The First Seventy Years (2006):
Delivered with serial 43-15749 on 04May44 to the USAAF, but soon -01Mar46- transferred to the Reconstruction Finance Corporation (RFC, see Wikipedia).
Its first entry in the civil registry was as NC20754 for David B. Oliver and operated for the Allegheny Air Cargo Inc., of Pittsburgh,PA (1947).
Next was Morrison Knudsen Co. of Boise, ID (28Apr54). In 1963 it was reregistered to N36MK.

John P. Stewart wrote, as soon as I published this page: "After reading the post about the Northeast DC-3 in New Hampshire, I thought I had shot the aircraft when it was in service with Morrison-Knudsen... So here is a photo I took at San Francisco (SFO) in -I think- November of 1965!"

12Feb69, as N364K, it was registered for Denver Beechcraft Inc and in July 1969 it was operating for Pacific Inland Nav. Co. Empire of Oregon.
Travelling north, to Anchorage, Alaska it was flown by Alaska Barge & Transport Inc (Oct.'69).
By May 1972 it was operating for Aero Retardant Inc. of Fairbanks,AK. And Pacific Alaska Airlines was next, in Nov73.
Entering Canadian registry as C-GYBA it was acquired by Aircar Services of Vancouver,BC.
Yellow Bird Air leased it for a period from 13Apr76.
Registry was amended for Skycraft Air Transport Inc. of Oshaway during May 1984.
Tt returned to the US again, initially registered as N347AB, then for the Dakota Aircraft Museum as N33623 on 17Sep93.

Online I found in July 2019 little or nothing had changed with N33623, perhaps not even moved from that spot!
Disappointing.

The following I found written on the subject at Ed "Bear" Hake's website www.centercomp.com (reproduced from an article written by Rand K. Peck in Airways magazine, July 2000 - and see his photos further down):

When airline pilots run headlong into the FAA's mandatory retirement at age 60, many contemplate the purchase of a small airplane. Most still enjoy flying, some see it as a mode of transport, while others enjoy the blend of camaraderie, cheeseburgers, and coffee found only at an airport...
The majority of these fellows will purchase a Cessna 172 or 182, although the more adventuresome will seek out something more unusual, such as a piper J-3 Cub or a round-engine Cessna 195.
Those who seek long distance transport may explore Cessna 310s or Beech Barons, outfitted with the latest GPS, moving maps, and other such electronic equipment normally associated with the cockpit of an airliner.

Then there is Mike Farrey.

Mike retired in 1994 from Delta Air Lines as a Boeing 767 captain based in Boston.
Coincidentally, his career had begun in East Boston in 1966 as a co-pilot on a Northeast Airlines Douglas DC-6B, only narrowly missing the opportunity to fly NEA's DC-3s.
NEA operated a fleet of 25 DC-3s over 26 years and operated its last DC-3 trip on December 16, 1966. The airline's DC-3 pilots had been 'frozen' on the type, thus Mike and other new hires went directly to the four-engine DC-6, pending the arrival of jets. The years 1966 and 1967 were busy ones for training at Northeast as the company took delivery of 21 new Boeing 727s, 14 DC-9s, and eight Fairchild-Hiller FH-227s.

I found c/n 20215, as C-GYBA of Yellow Bird Air, in my collection of slides. I purchased this slide from Per Schmidt at some point, unfortunately the slide had no info of where and when this image was taken. But suggestions poured in, read on below (Gary Vincent has been suggested as having been the photographer) Per bought the slide from someone else, unable to determine the name of the photographer. But Sean Keating offered the following suggestion: "I would suggest this photo was taken in Vancouver at the South Terminal... Note the mountains in the background! Taken during the days when you could park your car on the grass and just walk across the ramp to take a photo, and nobody bothered you! Given that the paint stripping is not complete, I first saw C-GYBA in 1980 and it was all silver with yellow striping by then, and the Twin Otter (Air BC or AirWest?) in the background may help date it..? My initial guess is late 1978 or '79." John Kimberley offered on this subject: "I would say the location is Vancouver South Terminal and I would suggest the date is late 1979 or early 1980. I have a slide dated August 1978 in similar colours but the tail was painted in Pacific Alaska colours. My 1st slide of the aircraft in this colour scheme is dated March 1980, shortly after I moved from Edmonton to Vancouver. However, I have two slides dated Apr80, 1 in that colour scheme and one showing the aircraft completely stripped of paint for a new bare metal scheme... So, in short (...) the photo was taken between August 1978 and April 1980! - but I would say that my more exact estimate above is not far off." "As a matter of interest, this aircraft was based at Innotech Aviation in Vancouver-YVR, where I worked on the ramp. I used to have a lot to do with it - fuelling, GPU, etc. I see in my log book that I had 4 flights in it: 27/12/80, at night from Vancouver to Penticton BC and return, to deliver some supplies because roads were blocked due to snow/flooding - 25/07/81 from Vancouver to Port Hardy and return - a passenger charter." Sean Keating wrote me on 03jan10 on the subject of C-GYBA: "I got a call from Glen Etchells, one of our local YVR experts; he photographed 'YBA on 15th March 1980 in exactly the same place and paint condition as your photo.. He commented that the paint stripping was halted for a period of about two to three months, but in May 1980 he saw it fully polished with new titles. The paint stripping was done by our friend Tyler who earned his living on many ramps in YVR cleaning and supporting commuter aircraft and bizjets. Given the level of inactivity of 'YBA, and the amount of snow on the mountains, it would be assumed that your photo was taken in that same time period, if not on the same day! I have added two of my photos, both scans of prints : 1. Vancouver in the winter, January 1982, 'YBA spent a lot of time here at the old Innotech ramp. 2. Abbotsford Air Show, Aug 1980. I think 'YBA was taking up the skydivers to open the show."