Sorry to break in. I have No.4 on the go now - but someone got to it before me and they removed the carriage rails, thereby losing the bearings. All else is there and I do like understrikes - so...
Could someone send me a photo of the bearings and any races? An approximate diameter for the bearings (and how many there are front and back) would be nice to know. I note that the front rail allows generous adjustment and have actually got it moving nicely with some bearings I had to hand (except for the bearings bunching and occasionally dropping out so I need to know how it should be).
Incidentally, the return strap was broken and I played with various materials before eventually making another that works well (and looks good) using 0.2mm phosphor bronze sheet. avs@nolandmines.com (that's the day job).