Looking at my list of cameras and in the closet I have found two Kievs that are very close together. I think this could be interesting:
First there is a Kiev III from 1955 with double engravings (latin and cyrillic) on the front mask that has the number 5511084.
Second there is a Kiev IIIa with also double engravings on the front mask that has the number 5511845.
Both have "Made in USSR" engraved in the accessorie shoe, so both are export models.
The difference between both is about 750 numbers. Princelle says the Kiev III was built up to 1955 and the Kiev IIIa starts in 1955. So I think the change could be within this serial number range.
What about your latest known Kiev III numbers and your first known Kiev IIIa numbers? Only export versions seem to be interesting here.