There's something a bit odd here, just looking at the basic dimensions the OP has given. Unfortunately, I can't get a clear image of the jpeg to check.

Gear diameters are 1" and 1.3125". I'm not entirely sure if this is the actual gear OD, or the gear PCD. For simplicity, let's assume PCD. 20T, 1" PCD, means 20DP. Sounds reasonable - common size. For the second gear, 30T and 20DP gives PCD of 1.5". So the centre-centre distance must be 1.25", not 1" as suggested.

Let's try in the opposite direction. Centre-centre spacing is 1"; 30T gear PCD is 1.5x 20T gear PCD, so we end up with smaller gear being 0.800" PCD; larger gear being 1.200" PCD. That's 25DP, as it happens. Doesn't sound like a standard size, but the gear diameters are nowhere near those given, especially the small one. I reckon that there's an error on the drawing (and note that I am using gear PCD here - the actual OD is slightly larger). To find the OD of a gear given DP and tooth count T, it's:

OD = (T+2)/DP

However, if you make that correction for gear OD, 25DP is very close to module 1, as per previous reply!