Ok, new discovery. I've been an idiot but for some reason I thought, when I first cast glance on the tubes, that the rectifier tube was a 506, probably because I googled and I found that it was supposed to be that tube and the shape and appearance was identical to that of the 506. However, now when I tried to study the 4 tubes to see if perhaps I could make a more accurate schematics, it turns out that the rectifier in fact is a R0437
https://www.radiomuseum.org/tubes/tube_r0437.html Luckily, according to radiomuseum it is "normally replaceable, slightly different" with among other the 506 tube. Shouldn't be a problem then?