Vol,
Since you already know who rechambered the drilling, that is no longer a question.Just for info, the fact that it was marked on the side of the lug would be a dead giveaway that something had been done to it at a pretty late date.I'm not going to sharpshoot what someone else did, but note that rechambering from X72R to X74R is considerably more than lengthing the chamber.In addition to the difference in pressures Kuduae noted, there is a considerable difference in head diameters(except for the rare 9.3x72R S&S)between the 72mm and 74mm cartridges.The 72mm is about .427" but the 74mm is about .465"in head dia.This means the chamberwall thickness would be a good bit thinner in the 74mm chamber.To save weight rifle barrels in this age drilling were usually made only as thick as necessary for the original cartridge,and may be too small for more modern cartridges.I can't support rechambering 9.3x72R rifles, except from older versions to the normalized version, which only removes an insignificant amount of metal.I don't reccommend you cut the drilling up, but you should at least handload cartridges appropriate for the 9.3x72R and do not use factory 9,3x74R ammo.
Mike
