So I've searched and searched for any information on this bike and I can't find a thing. Unfortunately all i have is the frame (without original paint or decals), the headbadge, saddle, serial number, stem, bars, and original cottered cranks. From what I've found online, this is the only miyata i've seen with this headbadge. And I can't find any information on decoding Miyata serial numbers. The original paint job (a sharp lime green) can still be seen on the threads of the fork. I do know that originally the bottom of the forks and the back triangles were dipped in chrome. I've included pictures of everything i have. Its not vital but I've searched for so long I just have to know now. Any help from you guys is much appreciated.

Serial number looks like B487779 (or B487719?).

The original chainguard bolts had their heads painted red. The chainguard looked like a pie plate with a series of circles cut out along the edge. Bars are Sakae. Originally two chain rings -52 and 40 teeth.