Hello Steve - here a a few leads...
http://www.aerofiles.com/_doug.html
Which includes-
C-53-DO Skytrooper
Serials
[41-20045/20136, 42-6455/6479, -6481/6504, -15530/15569, -15870/15894, -47371/47382, 43-14404/14405].
XC-53 1942 = Full-span slotted flaps. [42-6480].
XC-53A 1942 = [41-6455/6479].
C-53B 1942 = Navigator station, added fuel tanks. [41-20047/20050, -20052, -20057/20059].
C-53C 1943 = Side seats. [43-2018/2034].
C-53D 1943 = Side seats. [42-68693/68851].
Also-
http://aeroweb.brooklyn.cuny.edu/specs/douglas/c-53.htm
Serials
41-20045 / 41-20136, 42-6455 / 42-6504, 42-15530 / 42-15569, 42-15870 / 42-15894, 42-47371 / 42-47382
Probably the best site however for all such serials is Joe Baughers pages
http://home.att.net/~jbaugher/usafserials.html
Hope this helps
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