Digging back into the far recesses of my memory, I seem to recall that the TR250 and the earliest TR6s used the 15AC alternator which did have an external regulator. By 1970, the change over should have been well past completion to the 15ACR which featured the internal regulator. Officially, by somewhere in the 1972 time frame, the spares supplied for the 15AC were to use the 15ACR and punt the external regulator. By the same token, spare parts supply could get rather spotty in that early and mid seventies time frame. I recall many a TR250 and TR6 just sitting because you couldn't buy a TR6 brake master kit or cylinder due to a backorder issue. Dealers and outside shops were known to use whatever acceptable alternates out of their inventory or that they could get their hands on, then play mix and match to keep customer's cars on the road. It would not surprise me if a 15 ACR was replaced with a 15AC and the 4TR series (37588) external voltage regulator at some point if that was what it took to get the car on the road and they could their hands on it. After all, when MGB clutch master cylinder kits went on what seemed like terminal backorder in 1973, shops were using 1967 back MGB brake master cylinder kits and just tossing the extra bits until those kits too went on what seemed to be terminal backorder. Then it was on to using clutch master cylinders until they went on .... yea, you guessed it.
Having grown up around these, it just amazes me how much better and more consistent the parts supply is forty plus years after the fact plus given the time value of money, relatively speaking the parts don't seem to cost as much either.