OK - I can take a hint :-)
I’ve seen no evidence that the short blank (XM192) was ever adopted. The two examples I have in my collection are headstamped (+) FA 58 and (+) FA 59. Ray M is correct in stating that for blanks (and dummies for that matter) you can not rely on the headstamp to date the cartridge - empty cases that failed qc for ball, tracer etc production was often diverted to blank production. However, if the XM192 had gone into production I would have expected to see far more variations in headstamp unless it was only used for brief period.
In addition to the US cases, I also have an XM192? headstamped (+) DA 58 - Canadian production or US made on a Canadian case? I also have a blank similar to the XM192 formed using a commercial case - headstamp W-W SUPER 308 WIN - I wonder if this is a commercial blank, but can anyone shed light on this? Final cartridge in the photo is a short Candian blank headstamped (+) IVI 73 - again I’d be grateful for more information.
Left to right
(+) FA 59, WW SUPER 308 WIN, (+) DA 58, (+) IVI 73
As to the original photo, I agree with Ray that the prototype for the M82 was the XM82 and the example I have is heastamped FA. I don’t have an answer as to why there is a difference in overall length. However, in the early days of 7.62 NATO, blanks were formed from 30-06 and 8mm Mauser by a number of countries and qc may have left something to be desired!