Looking at the back of the RM4, the single lower tube is V1. The upper right tube is the output driver and the upper left tube is the loop buffer/mixer.
I spent the weekend rolling tubes in my RM4 and modules. I previously used an RM50H and figured they were different enough to revisit everything. V1 ended up being exactly what JF posted, a Ruby 12AX7AC5 HG, which is what it came with. I tried RCA, Mullard, Amperex, Telefunken, Tung-Sol RI, EH, JJ, Sovtek... nope, the Grail just didn't get there without that Shuguang in V1.
For fun, here's what ended up working in my stockish mods:
Clean - V1 RCA 12AX7, V2 Tung-Sol RI 12AX7
SL+ - V1 Ruby 12AX7AC5 HG, V2 Amperex (Hammond) 12AX7
Grail - V1 JJ ECC83S, V2 Amperex (Hammond) 12AX7