Sorry - I wasn't suggesting that the Toyota Starter Solenoids was necessarily swappable with the Triumph OEM one - just suggesting that the problem indeed was potentially down to the solenoid contacts and just relating anecdotal story of my own experience with a Toyota Starter. I do recall the posts where you swapped the Solenoid parts however.
You have to recognize these contacts switch a huge current and every operation causes
some damage to the contacts.
There is actually a machining process called spark erosion or EDM - Electro Discharge Machining.
@micromike1 can tell us a whole lot more about that!!