Yes sorry, I'm a bit late.
I had to study all the change history in manual 05335-90044, up to Serial Prefix 3154.
V1.1 had been introduced in June 1984, for all instrument with 2424A06001 and above.
V1.1 has one decimal point in the HP-IB string, as shown with my instrument.
V1.0 shows no decimal point in the GP IB string at all.
I already have the newer prescaler in the C-channel (05335-60034), which divides by 64.
The old prescaler (05335-60009) divided by 20 only, therefore the firmware has to match.
It's not clear, if there exists an intermediate firmware which corrects all the bugs of V1.0, but is made for the old prescaler, i.e. multiplies the result by 20 instead of 64.
The difference may be inside the EPROM A4U22, as 05335-80012 (as in my unit) refers to the new /64 prescaler, and 05335-80006 refers to the old /20 prescaler.
That's change 7 in this manual, and affects prefix 2820, exactly my counter, and also for retrofitting the newer C-channel option 030.
Obviously, I have an intermediate A4 version, 2 EPROMs - but V1.1 and the newer prescaler. Perhaps my instrument had been updated, or the C - Input had been replaced.
I could not find a comprehensive table, which EPROM code refers to which firmware version, and/or to which prescaler.
Frank