Hi Team,
Ron you are correct as per my 4G1--4G9999, form No 8326 Reprint May 1954 Parts Book.
BUT in the Index it says under Hour Meter see pages 61 and 213.

213 is in the Attachment section and it tells us that both a 2A5325 Grp. and a 5B8745 Grp. suits 4G1 to 4G 769 Inclusive.
Looking at the illustrations of the 3 Grps. 2 of the units :- the 2A5585 and the 2A5585 appear sealed and non-serviceable.
Likely differences other than the serviceability could be the gearing differences you allude to for maybe belt work.

All interesting stuff all these years later--if Owen T was still with us he would be pounding me/us for a definitive answer--RIP Owen T.
Cheers,
Eddie B.

Eddie, good work there, thanks. Who'd have thought to look in the index? Duh! I just flicked straight through to the hour meter page!
My book doesn't have a separate "Attachments" section. It does have "Attachments" in the index - but the hour meters aren't listed under that Attachment index!
The hour meters are listed under "Diesel engine" instead.

What it does appear, after studying page 213, is that the early hour meter was more of a sealed unit.
The later hour meter has a nut and gasket on the end with a retainer and spring under it, which allows for more disassembly than the earlier unit.
It's obvious the later hour meter comprises more separate components and is more repairable - which is the opposite to what you'd expect today!

The earlier unit has a footnote that, "if parts other than those listed above are required, send complete hour meter to Caterpillar dealer".
The early hour meter only has a replaceable seal, cover, pin and spring for the cover. Two components listed for the early model, that are missing on the later model, are two stovebolts and a guard, used for shipping and storage protection.
The later unit has a replaceable counter unit, a replaceable driveshaft bushing, and a replaceable driveshaft.

Going on appearances, the one that Garlic Pete has pictured is a later model hour meter, anyway - so it's not original.

Cheers, Ron.