Oh dear - that brochure brings back memories and sends a chill down the back of my neck. I bought one of Slater's last Egli twins in 1974, looked very much like yours then, I had many problems with it, but I've still got it so it can't have been that bad!
I think the later ones were built by a different frame builder than his earlier ones (I think 1970 was the first year he built them), but just in case, check the fit of the bearings in the headstock and swinging arm. My swinging arm bearings could be dropped in from about 3 feet, and would wobble down into position - the bore was too big, and someone had dot punched the bore to get a grip on the bearing outer - that probably worked for about 50 yards before the bearings came loose and the back wheel took its own route around bends.
Good Luck.
I think the later ones were built by a different frame builder than his earlier ones (I think 1970 was the first year he built them), but just in case, check the fit of the bearings in the headstock and swinging arm. My swinging arm bearings could be dropped in from about 3 feet, and would wobble down into position - the bore was too big, and someone had dot punched the bore to get a grip on the bearing outer - that probably worked for about 50 yards before the bearings came loose and the back wheel took its own route around bends.
Good Luck.