What you have is the top of line 1977 Miyata. In 1975 and 1976 it called the MX-P (P=pro?). The 1975 and 1976 are quite similar to yours, though they have chrome only of the dropout flats, there are nine holes in the bottom bracket and two round holes in each lug. They came only in orange.
Components are the same, though yours seems to have a more lightening holes and black anodizing. The frame material was 4140, double butted CrMo like yours. I haven't seen any source reference the manufacturer and this was prior to Miyata's development of their own foundry. Based on Miyata's later association with Tange, they are the most likely candidate.
It is a wonderful find! The top model from the best of the high volume Japanese manufacturers. Professional level and almost certainly hand made. I wish it was mine! Thank-you very much for posting and sharing.
Last edited by T-Mar; 05-31-05 at 04:09 PM.