Were the Mannlicher Haenel rifles built entirely by Steyr and just retailed by Haenel or did Haenal build them on Steyr provided parts? I have 2 of them, both in 9x57Mauser. One with the mag latch as on your rifle, the other with the magazine latch inside the trigger guard. #1 has a standard German sporter type bolt handle, the other a flat M/S type. I've seen a few for sale here and there, sometimes refered to as 88's, which they are not, one refered to as an 88/98 (that's almost a little closer!). Some info in the ALFA 1911 catalog reprint on the different grades and options but no history of course. The back end of the bolt with it's 98 style shroud and gas ring is interesting on these rifles as is the magazine assembly with it's sissors action spring.
