Having previously worn a flour sack (with one eyehole) in Part 2, Jason discovered his trademark hockey mask in Part 3, stealing it from practical joker Shelly (Larry Zerner). As any fool knows, the traditional Jason mask has three red triangles, plus an axe mark above the left eye where Jason was struck at the end of Part 3.
You can tell that Part 5’s killer is an imposter because his mask features two blue triangles (pointing downwards rather than upwards), lacks the axe mark plus any other noticeable signs of wear. Also, the actor who plays Imposter Jason, Tom Morga, is a mere 6ft 2in whereas Part 4’s Ted White stood 6ft 4in in his socks.
This is the only time that Jason has “shrunk” between movies. At 5ft 11in, Part 1’s Ari Lehman is the shortest actor in the role, eclipsed in Part 2 by 6ft 1in Warrington Gillette. CJ Graham (Part 6) and Kane Hodder (Parts 7-10) are both 6ft 3in whereas the two most recent performers, Ken Kirzinger and Derek Mears, stand 6ft 5in.