It's hard to deduct the amp and cab used on RTL from flemmins interviews :
On the first album we did together, Ride The Lightning we had a problem with James' amp. It was stolen a week before they entered the studio to start tracking. They were on tour when it happened. Now somebody in San Francisco had modified the amp but nobody knew what he had done. So what we did was we got all of the people we knew that played metal music and had good Marshall amps and cabs, we got them to come to the studio with their amps and cabs and we simply tried them until we found one that we thought sounded good
So we only know that he have to buy a new amp in July 1984. Could be a JMP or JCM 800 head, with probably Jose Arrondo modification and 6505 tubes as he have previously.
We have no information about the cab, except it Marshall 4x12 cab. The flemming notes http://www.fwrproduction.com/FWR_Produktion/Notes_1.html#10
mention just a 300 Watts Kabinet for the prince song, for Kirk but there is no 4x12 Marshall cab at this rating in 1985 according to the marshall book. More it's strange to wait the 11 sheet to mention it. It's look like more thant Kirk take a different amp for this particular song.
It could be a 1982 or a 1960 cab, even if 1982 are maybe more for heavy duty concert instead of studio.
If you go to the marshall history books https://books.google.fr/books?id=E90tMBs9_FEC&pg=PA90&lpg=PA90&dq=marshall+cabinet+celestion+history&source=bl&ots=ng3k-21iaJ&sig=5UwQq08fBLPRvj5z73ctLMjVdCA&hl=fr&sa=X&ved=0CCEQ6AEwAGoVChMI9rWDw-vJyAIVi1YaCh2yvgE2#v=onepage&q&f=false
, you could see that, between 82 and 85 you can get for a 4x12 1960A/B cabs only two speakers :
79-83 : G65M (Almost 80 variations of these was produced, it was a major step for Celestion in term of power handling, climbing from 25 watts to 65 watts !)
83-86 : G70M (next version of the G65, same magnet)
The GT75 was added later on to the Marshall catalogue. The speaker itself begin it life in 1984, so it's possible they have buy some of them to used them in a 4x12 cab.
86-90 : G75T (G12T75 production itself began in late 1984, magnet differ, type T)
If you consider the 1982 Cab, add the G12-80 and G12H-100 speakers. But i won't go either for a 50 oz magnet like the G12-80 or the G12H-100 as they tend to have a more nasal sound than the 35 oz magnets. You can get this difference by hearing any versus of a green back M/H magnet. An example : https://www.youtube.com/watch?v=du5sn1wtZWA
. the M magnet is really more scoped and better for classic trash metal chuncks than a H magnet in my opinion.
To have played both G12-65 (re-issue) and G12T75 (1984 and re-issue), i must say bass are really close to the MOP sound on a G12-65 and it particular high ends frequency signature is there. G12T75 also have spikes than you have not here and their bass are tighters. You can clearly get them on Slayers early records.
You can hear a bulk demo of a 1985 Metallica MOP demo https://www.youtube.com/watch?v=TYgroh_602Q
to get an idea of their sound unmodified and hear the Guitar heroes steam track of RTL and MOP to get an idea of isolated guitars sound.
Just my 2 cents. Cheers !