Re:
Maybe I'm pointing out the obvious, if so ignore me.
I bought a 17.5" Wild Cat Trail to build up for my wife but gave up trying to make a caliper mount adaptor to fit the Shimano post mount caliper, with 160mm disc, to the mounts on swing-arm that require the Marin lump of alloy adaptor. So bought a 15.5" Wolf Ridge with IS mount on swing-arm to swap over the arms. The swing-arm from Wild Cat Trail is longer than the Wolf Ridge one by around 20mm, would need to measure it to be sure. The Wild Cat Trail works fine with shorter arm, just less travel which matches the forks OK.
The 2 bikes LGF has posted pics of on 1st page have different lengths of swing-arm, the 1st one with what looks like the Marin caliper mount is the longer one. The 2nd V-brake only is shorter. The one posted by Matthews looks to have the same longer arm as the 17.5" Wild Cat Trail had, with Marin caliper mount.
The shorter arm, I just measured them, is 450mm from shock mount to drop-out, the longer arm is 470mm although I wasn't able to get as accurate a measurement with this one.
The 3 bikes my wife and daughters have are a 15.5" Mount Vision with the shorter IS mount swing-arm, a 17.5" Wolf Ridge with the shorter IS mount swing-arm and the 17.5" Wild Cat Trail again with the shorter IS mount arm. The 2 17.5" bikes, Wolf Ridge and Wild Cat Trail, are different in the other tube dimensions though and geometry too probably although I can't check this.
My plans are to find a longer travel fork, 130mm, to fit to the 15.5" Wolf Ridge with the longer Marin mount swing-arm so that the travel matches both ends, to build it up for me. Will be simpler to make adaptor to fit a Hope IS caliper with 180mm disc as the larger diameter disc gives more room for adaptor.