Coldkennels
Barnack-toting Brit.
Here's a quick comparison shot I did on HP5+ a while back:

I've used a bunch of Jupiter 8s and Jupiter 8Ms - although nothing like as many as Brian, obviously! - and this 1974 one is the best I've found across both mounts. The Helios is still sharper from edge to edge, with slightly higher contrast, and more jittery/interesting OOF areas than any Jupiter 8 I've played with. Testing on digital via adapter, it's sharper and has higher resolution than an undamaged and uncoated pre-war Zeiss 50/1.5 Sonnar at f/2, but I've yet to try it against a coated post-war version of the same.
The trade-off is that it's slightly longer than the J8M, and build quality of the barrel is noticeably not as refined or pleasant as earlier Jupiter 8s - but optically, it's a fantastic lens.

I've used a bunch of Jupiter 8s and Jupiter 8Ms - although nothing like as many as Brian, obviously! - and this 1974 one is the best I've found across both mounts. The Helios is still sharper from edge to edge, with slightly higher contrast, and more jittery/interesting OOF areas than any Jupiter 8 I've played with. Testing on digital via adapter, it's sharper and has higher resolution than an undamaged and uncoated pre-war Zeiss 50/1.5 Sonnar at f/2, but I've yet to try it against a coated post-war version of the same.
The trade-off is that it's slightly longer than the J8M, and build quality of the barrel is noticeably not as refined or pleasant as earlier Jupiter 8s - but optically, it's a fantastic lens.