I agree, the original relay probably does not need a diode. Mine lasted a long, long time with no diode; but it did finally give up the ghost after some 30+ years of service (and a whole lotta miles). The problem comes in with the replacement relays. My guess is that the original relay had special contacts that were designed to stand up to the kickback from the solenoid (which is a very typical 50s solution to the problem); but replacement relays (even repops) don't use the same material.
There are lots of different diodes that will work in this application. The one I used is part number 1N4004 (that's an industry standard part number, made by lots of manufacturers). I got mine at Radio Shack (back when they had a store on every corner); but here is another source
https://www.amazon.com/SODIAL-1N400...coding=UTF8&psc=1&refRID=7RXM39VBC5NVQVSG82YW
The diode will be connected between the relay output terminal and ground. Here's a typical diagram (originally supplied by Rob Michigan)
It does have to be oriented a certain way, which depends on how your battery is connected. If you look at the body of the diode, there should be a white (or silver) stripe near one end. If your car is wired negative ground (original for a 4A), the end with the stripe should connect to the relay terminal. If your car is wired positive ground (or earth if you prefer), then the end with the stripe should connect to ground.
Geo Hahn once posted a nice photo of his TR4 installation, but I can't seem to find it now. Perhaps he will post it again.