The photo caption on p. 42 lists serial numbers of six other 1914 DWMs in the 8000a--9000a serial range, the text on page 23 mentions them as well.
C/X is not a "regular" inspector's stamp. According to Görtz & Bryans (p.134) the letter surmounted by the crown was the first letter of the Inspector's family name. In the case of two inspectors whose names started with the same letter, the junior inspector would use a stamp with an uncommon letter, C/X or C/Y. I remember reading someplace (and cannot now find the reference, if someone could either confirm or refute this I would really be glad) that C/X was also used by new Inspectors who were on the job but had not yet received their own stamp.
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WAG SPECULATION:
By the 1914 production year the decree was in place that all P-08 were to be manufactured with a stock lug. At the time of the late a block production, frames which had been manufactured in a previous year, but not yet assembled into finished pistols, found themselves in the production line. As these pistols were technically "out of spec" they should have been rejected, but for some reason--probably having to do with the necessity to maintain production numbers--these pistols were appealed to some higher authority--the supervisor of inspectors or the head of the arsenal at Spandau, perhaps--who agreed to their acceptance. As this authority might not have had a regular inspector's stamp, the C/X was used in its place.
/WAG SPECULATION
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--Dwight