This Union Switch & Signal is one of the approximate 100 "EXP" or "Experimental" Series pistols that were produced. These pistols were assembled from trial receivers, slides and other components and were manufactured prior to regular production. These would be assembled by a small numbers of machinists that worked at the plant.
The observed examples of these pistols the EXP markings usually differ as they were all applied by hand and the placement will vary from different examples. Some of them are finished buffed and blued, some are blued and not buffed and some are buffed and blued with the top of the slide sand blasted. Some are parkerized and some are blued over sand blasting resembling a Du-Lite finish. The US&S slide logo is consistant across the observed examples.
EXP No 5 is fully buffed and blued and utilizes a pre-production receiver. The EXP No 5 is partially hidden under the right grip panel and can be observed once the grip panel is removed.