Thxs. That's a common serial number format, so I'd agree with 1976, either either June or the 6th fortnight. Unfortunately, I haven't seen any D-codes using this format, so I can't identify the manufacturer.

Given the 26.8mm post size, there is a good probabitiy that that is actually Tange Champion tubing. A Tange steerer would increase the confidence level to near absolute. Unfortunately, unlike most other tubing manufacturers, Tange used double double butted seat tubes and it was a thickness for #1 , #2 and #3 . To make things worse, the top butt was same thickness as plain gauge #5 ! Still, the most likely candidate is Tange #2 , as it closest to Ishiwata 022.