There are 4 types of rear arms about, the thin crudy useless things fitted in the 90's as a cost reduction, the tubular type fitted from about 1986 when they went over to M20 rather than the finer imperial thread and on some 110s a very heavy guage tubular type. Oh and of course the early pre-EFi RRC type which you don't want as the threads are a weak-point.
The only ones available now are the tubular M20 type which is fine as they are what you want.