As Justin says Brevet is just French for Patent, so nothing abnormal there and it appears the 'G Monnin' stamping was done once received by the manufacturer based on the pieces I've seen and my own one.

ESA movement would be correct for the early Monnin produced watches.

Heuer historically were absolutely atrocious at recording anything about the watches made. I have no clue in most cases why they even bothered with serial numbers!! So small changes here and there is quite common.