Thanks, Dave and Billca.
The service manual/driver's instruction book says "fill the radiator with soft water (e.g. rain water)"! The model was renowned for its not being possible to boil it, so any improvement or degradation in cooling from using something in addition to water is not an issue. I'll check into what Kragen stocks, but the usual problem with additives is that they don't tell you what they are; they merely make probably spurious snake-oil claims.
Yes, someone somewhere suggested that the various metal components would be protected by very slight alkalinity. Someone else indicated that it was unwise to put glycol into engines that predated its use, but I am not sure what damage if any it might cause. (When I took over the car a year ago it contained antifreeze because that was necessary in England).
At present I am likely to be draining and refilling several times, as I chase minor leaks, so I am not prepared to put in expensive chemicals.
Ken G, 1925 Rover 16/50 (San Francisco)