Yeah, it's an 1820-51. Used on the 'A' and 'B' models
1) The prop governor is at about the 1:00 position, and the "wheel" on the prop governor is to the left (looking from the front). On the -97, it is closer to 12:00 and the wheel is on the right side.
2) The hose coming from the nose case at the 6:00 position is where the front sump would be located on the -97.
3) The ignition harness is located between the pushrod tubes and the cylinders. On the later model -97, it is forward of the pushrod tubes.
Still investigating to see if there are external differences between the 1820-65 used on the 'C', 'D' and 'E' and the 1820-97 used on 'F' and 'G'. I think there may be a difference in the prop governor pad, however; looking at the 1820 parts manual, it may be just be an unrelated change in design. Lots of interchangeability between the -65 and -97.
And, yeah, I wish I would have recognized this earlier during the great Mary Ann hunt. She definitely has B model 1820-51 engines. But now I understand why there is a difference in detail in plastic model engines.