Soo good news and bad news...
the service manual I have does NOT give a definite answer to your question..
it only refers to the NO-LOAD rpm to be taken from another manual

the single time it lists NO LOAD rpms is for fixed speed generators
(3krpm-50cycle 3150rpm, 3.6krpm-60cycle 3750rpm , both regulated)
BUT all testing on the alternator (and most testing of the oil pressure) is done at 3600 as far as I can see ...
So yeah 3600 is still the value I'd go by.
'sid
ps:
the manual... in case I misread or simply missed something:
https://rich.homeunix.com/gravely/pro16/272144 Vanguard Twin Cylinder OHV BRIGGS & STRATTON.pdf