You were free to order whatever sizes you wanted. I don't recall 12/16/20 being a standard Slingerland catalog outfit (generally you'd see 12/14/20) but it's possible in the fine print of the description there was an option to get the 16 and irregardless of that it would be easy enough to tell your dealer that you wanted that size in place of the 14. Rogers did catalog sets in that configuration so the idea of that combination of drums was out there and being used by many drummers.

Slingerland's first catalog kit with 12/16/20 was the Cusatis 4N. It showed up in the '69 catalog, meaning 1968. Chances are that it was available earlier. The '69 catalog also mentions that you could substitute a 9x13 for the 8x12 at no additional cost. I had this exact kit, with the 9x13.

As K.O. pointed out, you could order whatever sizes you wanted. The bottom line is that many drummers made changes to what was shown in the catalogs to suit their own taste. What this boils down to is that your kit most likely has no corresponding catalog name

Sets were often pieced together out of orphan drums on those days, like my first Slingerland set; 13" and 14" toms and a 20" bass drum. Those were the sizes my drum teacher had in stock. I should have had a 16" floor tom, but who knew back then? I was just glad to have the drums. We weren't so hung up on sizes back then.

Thanks all for your replies. I kind of figured what you guys suggested was probably true. No big deal, just a curiosity. They were built so well back then, I don't need a kit name, they sound so wonderful Cool Dude