In fact, initially to accomodate the breakages that started to occur on Forward Controls because of the weight, there were changes from mid-60s. First attempt was a very small machining change to the radius of the flange corner, then later there were improved grades of steel used. I think by Series 3 LWB there were no more changes. Yes, for bonneted LRs they are interchangeable, but no way on earth would I use a 54 version on a FC. The best practice is that you can use later ones on an earlier LR, but not use early on later, because of gradually increasing weights. They do break, although I admit never seen a catasrophic failure, just hairline cracks. I explained the steel grade changes in an earlier post in the Forward control threads.

