I saw 51 MON in the early 70s, but at that time it was on a Dutton, which was a Lotus 7 style kit-car that was based on a Herald chassis. Must have been a Triumph nutter. Or a man with access to cheap welding (for those who know the Herald’s appetite for salt).
Why do I remember? Because we were both broken down in the same lay-by one hot summer’s day, me with my Herald 998cc twin-carb convertible.
Around that time I considered changing my name to Mike Kenneth Oliver 198 Fredericks, which would have got me a personalised number for free.