I was still at school in 1972 but I can think of a few reasons why they wanted you to add this can of Sulphurised Ester.
The first reason is they might just have wanted you to add Ester to the oil as a 'synthetic' base oil. Remember back then, Group I base oils were the norm and early Group I 20W50s wouldn't have had a great deal of resistance to oxidation. However most esters have brilliant resistance to oxidation. The problem with all esters is they are extremely expensive and most of the oil marketers would have been highly resistant to putting them in run-of-the-mill oils. Getting the owner to buy the ester separately might have simply been a way to get around this.
The second reason is that they may have viewed the Sulphurised Ester as an Antioxidant additive. I've never used them as such but I have plenty of experience using Sulphurised Olefins as Antioxidants (in both cases, you're simply using H2S to saturate a double-bond on an backbone). I can't say I'm a great fan of Sulphurised Olefins AOs (they're very smelly and aren't great on seals) but if you're chasing TEOST in a heavy Group II oil, they're often the most cost-effective additive fix.
I don't think Sulphurised Esters would have be seen as an Extreme Pressure fix. You might use something like Sulphurised-PIB for Gear EP but absolutely not in PCMO.
I'm not sure what to recommend for your Plymouth. I would deffo stick with 20W50; don't go thin. The trouble with US 20W50s is they're not very good for what you want (Group II with low ZDDP). What you really need is a nice Middle-East Group I 20W50 SL/CF oil (or better still 20W40). Strangely something Iranian might be perfect for your needs!