Long Branch Experimental Serial # EX2 ....

[Thunderbox]

My own view is that the "EX" marks were added some time after the receivers were made: the receivers are well-finished (at least EX11, which I examined in some detail), whereas the EX marks are roughly struck. I'd guess that the receivers were pilot production or gauging samples from the (then) new Long Branch production line, and that they were initially stored or held as reference samples. Later on, they would have provided a convenient pool of top-quality receivers suitable for use as test-beds in trials or experiments.