I've recently been spending a bit of time on customers' early wristwatches, both prefix series and pre prefix models. These earlier watches are hand finished to a greater extent than the later ones, with carefully poised balances. These do not readily interchange between watches. Not only are they all worn to much the same degree through use, but they have often been subject to filing and other attempts to alter the timekeeping. Some of the hairsprings lose their tension over time which doesn't help. I just finished an attempt on a pre series that had had the balance bridge changed, presumably with the balance as well, after the repairer found that the balances aren't readily interchangeable. As a result of the repair the watch is a poor performer with sub 200 amplitude readings. Attempts to fit a later style balance proved fruitless. Inspection of a quantity of similar aged movements showed that swapping balance bridges resulted in misalignment of the balance staff in many cases. Because the example I was working on had, like most of these early watches all major parts numbered for identification by the assembler, the obvious conclusion is that the watches need to have matching parts for sensible results. By the time the mass produced gilt e series movements were in production, parts were truly interchangeable, at least within reason, bearing in mind that balances are different on the shock and non- shockproof later watches. Incidentally higher quality balances can be found on most of the better Benson versions, these balance assemblies are similar in build to the early Smiths ones with proper timing screws.