I thought you would have more input with regards to this problem. When I did do stock re-finishing and repairs the broken but plate seemed to come up with regular monotony.
My method for repair I found worked on the three types of but plates I found in common use Horn, Plastic, and Vulcanised Rubber.
The one piece of equipment I found indispensable for this repair is a “Dremel” though I would never pay what I think is an exorbitant price for one so I use cheaper copies, with the logic being I can purchase three cheaper ones and destroy them slowly for the price of one Dremel. But let’s not get bogged down on that!
In the packs of miniature accessories, you will find glass mat construction cut off wheels, they do vary a little but the average thickness is about a millimetre. Using one of these wheels I cut a channel along the broken edges trying to keep to the centre covering about three quarters of the break length as deep as practicable, but do not go to an exposed edge. The repair consists of a length of steel strip epoxied in to this channel on each side of the break, if alignment is a problem open one of the channels to allow a little wriggle room. align the plate back by placing it on a flat surface for gluing using an Epoxy Resin Adhesive I Also put a thin polythene bag between the flat surface and the plate to stop the obvious, this type of repair is invisible when done correctly.
Another method is to grind away a recess either side of the break on the back of the plate deep enough to embed a metal splint in Epoxy covering the break. This method is a lot quicker but not as stable as the first method, sometimes the crack can open and show but the plate still stays together.
My source of thin metal for using as splints is Automobile Engine feeler gauges because on this side of the pond they are very cheap and you do get a lot if varying thickness splints for your money.
