Hmm. Interesting.
My two cents worth....
The first type, thin lip cover, i believe, we've identified being on E-vins up to 2134. The boss and the lip is thin all the way around the outer periphery of the cover.
The lip seen on your bead blasted cover is the cover which, in my experience, followed the thin lip cover. 4363 has this type cover, which i've always thought of as the "fat lip" cover, the boss is "fat" as well as the lip on the lower outer periphery. (Thin lip above the "fat" boss.)
The 3rd type, which you refer to as a "fat lip" cover, has two "fat" bosses. My memory seems to tell me that type of cover i've seen on bikes such as the K4-K6... ? (It's been a long time since i've been up close to a later 750.) To the best of my knowledge, this "double boss" cover was not fitted to any sandcast from the factory.... Or, "K0"..... ?
At least, that's my opinion... Someone more familiar with all models CB750 could clarify.
ps - For what it's worth, the parts books from 4.69 through the K4 book of Jan.74, the part number for these covers is the same; 11631-300-040. Beginning Feb.71, the book lists two gaskets; 11397-300-030 for vins 1-31370, beginning vin 31371 the K1 book lists pn 11397-300-303. The K2 book lists the 030 gasket as being superceded by the 303 gasket. The K4 book lists only the 303 gasket.