according to the shown link, the type c of FED-1s come in three sub-variations.
however, i doubt if the serial numbers match in exact order with these categorization.
that is, i have seen a specimen on the web which has a serial number of approximately... 81300, though it had its 'top front center screw' 'totally visible', and found the grouping system of sovietcams quite doubtable.
well in general Aidas's (sovietcams.com) grouping is based on the samples currently tracked and available to him. As we have seen over and over in Soviet camera production, there are always a lot of inconsistencies especially dealing with pre-war FED-1 production as sometimes old parts were used in newer variations or newer parts in older variations during production, and often cameras were repaired using all kind of parts, even from Leicas. So finding one deviation does not necessarily invalidate the classification.
Vlad is right when saying that repairing can change one or several typical features... More: I think that now some sellers reconstruct cameras from spare parts of different periods: it's particularly visible for the 1b-s and 1c-s. For example, I have already seen a cover of 1b used on a later camera... Much better for the sale!