Joe,
As to the other designations, the various Russian index code lists that can be found online are populated from manuals, markings on items, magazine/book references, etc.. So unfortunately you can never be 100% sure of their accuracy (they are generally pretty good though).
I have no manuals covering the 1910/30 (Model 1910, modified in 1930) howitzer, its GAU index code 52-G-462 (hence the projectile designations with 462 weapon system code in them). Most of the data used in the Russian Wikipedia page on the M-1910/30 seems to use a series of articles on the history of the gun that were published in the 2014 editions of the Russian weapons technology magazine 'Техника и Вооружение'. One of the series of articles (2014, Edition 6) does list the 53-OF-462L, and it seems all the other ammo used by the gun.
If one of the Hungarian manuals lists the 53-OF-462R, then it's kosher. Though it's description is almost exactly the same as the 3OF7 and 3OF8 designed for the AR-30 fuze. Maybe the 1910/30 had its own version taht used the AR-26 fuze, maybe the 53-OF-462R was dumped with the arrival of the AR-30 fuze, or maybe it got slightly modified for the AR-30 and then redesignated, who knows. As I said, I don't have any manuals covering it, or the AR-26 fuze.
Neil