I'm not entirely sure this is an M lens - according to my memory (and Pentax lens guide) the first time a 135 mm lens appeared as an f2.8 was as an SMC Pentax-A lens from the early 1980s. This puts it in the 3rd generation of Pentax bayonet-mount lenses (K, M, A). Assuming this lens is is excellent condition (don't buy a lens with any faults whatsoever, there are plenty of really good ones to choose from), $79 is a reasonable price and the SMC-A series lenses work without restriction with the auto exposure modes of later models (except of course they need to be focused manually when put on autofocus bodies).
PS: There has never been a "lower grade" of Pentax prime lenses (fixed focal lengths). All of these are excellent quality by the standards of the time when they appeared. The same does not apply to Pentax zoom lenses, the quality of which ranges from quite good to very nasty (some of the Pentax SMC-F autofocus zooms).