The short-lived 6501 by MOS Technology appeared as far as I know only in a board made by The Digital Group.

Aroudn 1975 The Digital Group created their new, reasonably priced, multiple-board computer system with these advanced features:
– Intel 8080, Motorola 6800, or MOS 6500 CPU card
– video display/cassette interface card
– 8-bit parallel input/output ports
– 2KB dynamic memory
– The 3-board Intel or Motorola CPU kit cost $425, while the MOS 6500 CPU kit cost $375. Later, the Zilog Z-80 CPU was added to the selection for $475.

The Digital Group didn’t just copy the MITS Altair design as others did, they improved upon it, with features not available in other systems. For one, the DG system loads the Operating System from the cassette drive automatically with little user interaction. Additionally, the DG system was available with four different CPU cards – the rest of the system remains fully compatible, no matter what CPU card was installed. Technically, the DG system has three different buses for CPU, memory, and I/O. Because of this, the card slots are different sizes and dedicated to each card type:

 650X CPU card construction PDF
 650x Operating system PDF, with listing in octal!