So, I have it confirmed: a "Profile 2" bayonet (Swedish AK4 bayonets for sure, likely Danish AG3 "HMAK" marked, and perhaps some others) are needed to lock up properly in the early-model HK91s that have the hole in the sight block at 9'oclock. An 'ordinary' newer model bayonet lug can be used. When screwed in with the threaded hole at the 9'oclock position, this adapter places the hole for the lug on the bayonet at about 1'oclock or so - which is the correct orientation for these bayonets.
Later-model HK91s have the hole for the bayonet adapter at around the 4'oclock mark, on the right hand side of the gun. In the pics, you can see that the early model HK91s are oriented entirely differently. Later models will require the more common "Profile 1" types.
On a side note, after all the confusion, back-and-forth and nonsense .. I'm kind of happy that this old '91 requires a weird bayonet. The stainless Swede bayonet looks pretty sharp (no pun intended) with the wood furniture!
Awesome--- you may want to post just the preceding post in "The Reference Library" for future bayonet users!!!