Though little-spoken about, The Severed Arm is a pretty solid proto-slasher. Certainly it's a bit dry at times (name 70's horror flicks that weren't), but the story is pretty fun, and the film possessed a solid atmosphere along with a rather enjoyable, if not prolonged, finale.
Have you seen this early 70's film?
(http://fight-evil.com/movies/years/1973/292-1.jpg)