T-Mar

I can appreciate why you would think that it was a 1979 model, given the crankset and centre-pull brakes. The component mix is definitely 1979. However, the frame characteristics are a better match for 1980. It has the fully chromed 1980 fork versus the half painted fork of the 1979 model and the rear gear cable housing stop is brazed under the chainstay versus the top mounting used on the 1979 model. Based on the serial number number and frame characteristics, it would appear to be an incorrectly rebuilt 1980 model, unless it is some strange foreign market variation. The former seems more probable.

T-Mar

Metric Reynolds 531 butted seat tubes typically require a 26.4-26.6mm post. Your post size is significantly smaller than what I would have expected, even with a distorted seat tube. The cinch slot appears to be almost pinched shut in the early photo, so the oval shape appears to be due soley to the undersize post. In the 2nd photo of the seat tube opening, it looks like you've pried the cinch slot open. I'd just deburr the inside of the tube, then pry the opening a little wider, so you can test fit posts past the bottom of the cinch slot, where there should be no distortion. Tightening the cinch bolt with the proper size post inserted should restore the shape of the seat tube.

That post scoring is from more than just and overly tight cinch bolt. There's a substantial burr or foreign material somewhere inside the seat tube. If the scoring is on the back of the post, the culprit is typically a burr on the cinch slot. Scoring on the front of the post is generally from a burr on the vent hole. Poor deburring of the tube opening or foreign material can cause scoring anywhere on the post.

JohnDThompson 

The dropouts look to be later production Simplex, which were threaded and had stops to acommodate non-Simplex derailleurs:

martl

Absolutely. I found several on very nice upper-mid level bikes.