Greist made buttonholers for lots of brands and as I'm sure you know, and different models have their particular fitting for the presser bar screw. In addition there's high shank, slant shank and low shank. The 185K is a standard low shank. The attachment should fit pefectly into the square part of the presser bar and the screw has to be tightened well. The feed cover plate has to be the right one too of course, but as long as the needle hole is in the clear it's likely fine.
There are several types that should work just as well for your model, the black Singer buttonholers with or with out templates, as well as the newer with white or colored plastic cover. There is a couple of a later version with extra large plastic cams, I think there's a low shank version for it too.
Here's a good post on various types, most of them come in a low shank version.