John;
Hi!
OK - best I know, somewhere between 254 and maybe 335 or so StuG III F/8 were built between SEP and DEC 1942 - being succeeded by the G in DEC '42. (references offer different production numbers!)
NO F/8 would have been "factory zimm'd" - that order did not go into effect until SEP 1943, long after F/8 production.
Refurbishing of tanks and StuG (by Nibelungenwerk, etc.) included zimming some older vehicles - but only after about OCT 1943. So a refurb'd F/8 MAY have been zimm'd, if refurbed after SEP or OCT '43. But how many were?
As for "field zimm" - well, I cannot say it did not happen to an F/8 - but there seems very very little photo evidence of any F/8 with zimm at any time, so there is likewise little compelling support for a field-zimm job.
I would say that you are quite safe NOT zimming an F/8 - in fact, I would be skeptical of any zimm'd F/8, unless someone could offer up some photo showing such! While I am convinced that some F/8 somewhere, sometime did get zimm'd, it would, I bet, be a rare beast indeed!
Cheers!
Bob