Hello wce
Looks like the carburettor is a probably a G2 (3-260). It has a large metal flag and a slightly different governor arrangement, but the same governor vane gear and pinion as the G3. If you do remove it from the mower I'd be keen to see a photo, especially of the governor spring.
Given the engine date and the G2, it does seem to correspond with a Mayfair 68-69 Deluxe 2 Model 559 (V76). The chassis base CH81648A (8-1648) does have a slot (drain?) towards the rear, as per your photo.
Mark
