There is a very interesting Penton frame on Ebay. It is listed as a 1979 but is clearly a 75-76' vintage frame (painted) but without the Hi-Breather backbone. I have never seen such a frame. I thought all bikes that vintage had the hi-breather set-up. Could be the hot way for sombody running MX. The number is: 4634572526
Chris
From the frame number posted, it looks like a July of 75 production date.
Lew Mayer