Since this is unidentified I can't say it is not MARS, but I really doubt it. The callsigns are not typical of what might be heard on MARS freqs, and the traffic there has never appeared to be a MARS net of any kind. Indeed this example is unlike any MARS traffic I have ever seen, and the use of AM for data, voice, and CW would be unique to only this freq if it was MARS, pretty unlikely.
The SIGINT training thing gets thrown around whenever there is an odd signal like this. Again I am skeptical, but less so than this being MARS.
With all that skepticism out in the open, and working against a couple of options, I have no strong ideas as to what it could be. I am inclined to think training, but not specifically SIGINT training.
T!