As I thought, that distance would make it an intermediate action that's been opened up to fit the longer cartridge. I believe they were produced by Zastava, likely using acquired FN machinery that had been used to make the 1924 military FN. The actions were modified for no thumbcut, no clip feed and the bolt handle was modified similar to commercial FN with flat bottom. Zastava also produced a similar action that had a mannlicher bolt handle (V550 or V500?) and was sold by a company in California in the 1960's. A regular standard length bolt will not fit. A Yugo 48 bolt may fit as it was, I believe, basically the same action.
I am still not sure that Heym used FN actions. I've read comments by people suggesting they did, but their description leads me to believe in most cases they are describing Zastava actions that look like
FN actions. I am almost certain that FN commercial actions were all H ring, not C ring as made by Zastava. I'd really like to see a picture of a Belgian FN marked Heym as well as a ZG47 Heym.