D8 would indicate it's a D1-8 mount. Yes D1-8 spindles use 6 cam locks but not all D1-6, D1-8, etc accessories will always utilize all 6 pins. Actual jawed chucks most likely yes but if it's something lighter duty there is no benefit to use all 6 cam pins. Quicker to just use 3.
It does look like it would be D1-8 also judging by how large the nose taper register is compared to a D1-3 or D1-4.