This is most likely a process lens, not a standard camera lens mount. These were commonly used in large reproduction cameras for copying artwork and documents in print shops, newspapers, and publishing. Rather than using a normal interchangeable camera mount, they were often attached to a wooden or metal lens board with screws—so the six-hole ring is likely for board mounting.

It was probably used on a large-format process camera rather than a conventional view camera, though the mounting method is similar in concept.

Adapting it to a Canon or Nikon DSLR is possible, but usually requires a custom setup rather than an off-the-shelf adapter. A practical method is to mount it to the front standard of a bellows and bolt it to a plate or board. Because a 250mm lens typically needs about 250mm of flange-to-sensor distance for infinity focus, you’ll need a lot of extension. Extra tubes may also be needed.

So: yes, adaptable in principle, but only with a custom bellows/lens-board style solution.