Hi (various people interested in this subject).
I have spoken to a few ex SSL people so far and they can't remember exactly where the 651 rails were bought from but I have a few more to contact and will let you know as and when I find an answer so, bear with me on this.
I guess it was too long ago for them to remember (about 40 years) or that they were not involved in the buying process but someone, somewhere will know and I think I have enough old contacts in order to find the answer hopefully.
I will post when I find out more but I still think that, as the sliders were all metal, it may be worthwhile taking the console fitted slider out and adjusting the movable bit that rises upwards in order that it stays in position, otherwise, it could be that metal fatigue has affected it after many decades. I do remember that, even though they were new at the time in the late 1970's, one end of a 651 would sometimes fall down and may need the slider assembly adjusting.
Hope this helps for now and regards from Jim Lassen
It may be just a case of applying some WD40 type stuff and/or adjusting the position of something within the slider.