You might have the Thrift version A15. I suppose if you take the rocker cover off and check if it has '13' on the head bolts this is a pretty consistent indicator of a thrift engine, but you made ought to wait for some other advice. All the thrift engines I have had (and seen) have had the 13 bolts.

That isn't a biggie as it helps (as you said) with the lower revs.

It certainly sounds responsive.