The C53 Skytrooper and C47 Skytrain were purpose built Military Transports with strengthened floors, glider towing cleat etc and both with more powerful engines ( 1200HP P&W 1830's) compared to standard civil DC-3s with 1100HP Wright 1820 engines.
Primarily one is a Troop/Paratroop Transport with a "passenger" door, and one is a Cargo Transport with Cargo doors.
Many more C47's (10,048) than C53's (404) were built due to the versatility to carry both cargo or troops (on side mounted benches) and later models of the C47 had twin stage supercharger P&W 1830-90Cs for use "over the hump".
Many other "C" definitions from C41 through to C52 covered "civil" DC-3's either ordered directly or impressed from airline production or service into the USAAC, most of these were Wright powered, and all with standard passenger doors and airline seating- and many with the pre-war right hand rather than left hand door, all as stopgaps before the C53/C47 Military versions rolled off the production lines in volume.
Assuming the tail number of the CAF a/c is correct, it is c/n 11757 C53-D 42-68830 delivered on 7 July 1943 and later flew post war as NC45366. Built with P&W 1830's it was converted to Wright Cyclones (1820's) in the 1980s by a private owner for use for sky jumping.
as per the site,
Some interesting information about the Douglas C-53 Skytrooper: "221 of these troop-carrying transports were built at Santa Monica, California, and most were powered by Pratt & Whitney 1,200 hp R-1830-92 engines. (Ours has Wright engines on it.) They did not have the large cargo door, reinforced floor or astrodome of the C-47s. They were fitted with 28 fixed seats and a towing cleat for use as a glider tug." (From the book A Celebration Of The DC3 by Arthur Pearcy.)
Additional facts:
C-53 : basically a troop transport version of the C-47, with side seating for 28 troops and a port-side passenger door. But no large cargo door. Total of 219 of this version were delivered.
C-53B: winterized version of the C-53, with extra fuel capacity and separate navigator's station; 8 were built (42-20047/50, 42-20052, 42-20057/59).
C-53C: same as C-53, but with a larger port-side door; specialized as a troop transport and glider tug; 17 were built (43-2018 thru -2034).
C-53D: same as C-53C, but with a 24-volt electrical system. Total of 159 were built (42-68693 thru -68851) at the Douglas Santa Monica plant. Ours is included in this batch - USAAF Serial #42-68830.
The above information comes from the book:
The "C"-Planes, U.S. Cargo Aircraft 1925- to the Present, by Bill Holder & Scott Vadnais (1996). ISBN 0-88740-912-1.
also see this ANAM DC-3/C50 website
http://aarg.com.au/DC3.htm
regards
Mark Pilkington