Long life to all the Feds!
But I don't totally agree with you when you say that your Fed is a transitional model. If I try to summarize:
-The Fed NKVD 1b are engraved "YCCP" with serial numbers from c. 6000 to c. 55000 and round covers.
-The 1c show the same engravings but with cornered covers. Probably to put the mechanism of the 1B, as you said. Serial numbers: c. 55000 to c. 92-95000.
-As for the 1d: CCCP engravings, serial numbers till c. 176000 and two other changes compared to the 1c:
1- the central screw half hidden behind the lens rim is now completely visible,
2- the three screws under the rewind knob are no more visible.
There are transitional models between # 92000 and 95000: Fed gradually introduced the two above changes. And it seems that the second change was introduced the first and the new CCCP engraving was the last. To be more complete, there was between c. # 80-82000 an other modification: the base plate pin was enlarged to 5mm (from 3mm).
All that seems perhaps quite complex, but it's nothing compared to the multiple evolutions of the 1b!
Amitiés. Jacques.
PS: so I think that your camera is a plain 1d.