Engine number? I notice it has battery ignition but no starter and no tractor style generator. Literature indicates it is crank started. It must have a battery for ignition power. Does the main generator recharge the battery? I also note no ZIP code on the tag. So it is pre-1963 or so. This model is shown in a catalog from 1951.
The model designation breaks down as:
R - IH gasoline engine
60 - cubic inch displacement
A - alternating current
18 - rpm in hundreds
They also made a Cub powered unit that ran at 1200 rpm and produced 3.5 KW rather than this one's 5.0 KW. Looks like it can produce either single or 3 phase (5.0 KVA or 6.25 KVA).
If Harvester had owned the company, there would have been an "IH" logo on the plate.