Howdy Harvey and All:
What a lovely mystery!!! We have evidence of 200 blocks appearing as early as 1962 in the Fairlane mid-compact line. The casting was C2OE-6015-C. It was a 4 main crank.
The '63 & '64 model years of Fairlanes showed up with a 4 main 200, casting codes C3DE and C4DE. The first 7 main 200s begin to appear in the '64 model year in Fairlanes and '65 models of the Falcon, Comet and Mustang with a C5DE - H castings. All we've seen, so far, have been 7 main, 5 frost plugs.
It is very difficult to be definitive in this time period (Probably any period with FoMoCo). Changes don't seem to follow dates, codes or guides.
Harvey, you've found a very interesting piece. Please explore it and share your findings.
Adios, David