The MCS6501, a first and very rare member of the MOS Technology 65xx CPU family.
Introduced in 1975 with the MCS6502 to MCS6505, pin compatible with the Motorola 6800 and, after legal issues, not produced afterwards.
An advert in the magazine Electronics August 1975, the price is incredible low for these days at $20.
The (preliminary) datasheet of August 1975 is the only document where the 6501 is documented. PDF format, Below the pages are shown.
More photos found on the web, the 6501 is at least produced in week 34 to week 50 of 1975.
Afaik the only known system where the 6501 was used is the The Digital Group 6501 CPU board.
The following table summarizes what makes the 6501 pin compatible with the 6800. Note the two phase non-inverting clock, Data Bus Enable
| Pin | 6800 | 6501 | 6502 |
| 2 | Halt | Ready | Ready |
| 3 | ∅1 (in) | ∅1 (in) | ∅1 (out) |
| 5 | Valid Memory Address (output) | Valid Memory Address (output, always high) | N.C. |
| 7 | Bus Available (output) | Bus Available (output) | SYNC |
| 36 | Data Bus Enable (input) | Data Bus Enable (input) | N.C. |
| 37 | ∅2 (in) | ∅2 (in) | ∅0 (in) |
| 38 | N.C. | N.C. | Set Overflow Flag |
| 39 | Three-State Control | N.C. | ∅2 (out) |
