Ok, I've been wrestling with Photobucket and seem to have their slightly changed arrangements pinned. Thanks for your photos and offer Grouty. Nice bike you have built and very much along the lines of what I have in mind.
My B50 shown came with the lay down shock modification to the frame which has been powder coated.
I've spoken to CCM/B50 replica frame makers Joe Maxwell and Simon Cheney about their frames and had a look at a couple of other lesser known and IC efforts.
Joe Maxwell informs me that his CCM 1974.5 bikes have 6.5 to 7 inches of rear travel, so not kosher for Pre75. A Cheney, or replica, appeals; low, light and simple and with the winning history and no fuss design approach.