Thanks for the helpful information - I particularly like the hazard flashers analogy!
I also asked the Tank Museum who confirm what you say, as follows. 'The PASS sign was painted on the reverse of the removable Arm of Service plate and was supposed to be used to indicate that the vehicle was halted due to breaking down, indicating that other drivers should go past rather than wait for them to move. As such, it should really be fitted to the rear of the vehicle as a traffic control measure. The reverse of the plate was supposed to be Green G3, though Black was also used at times to give better contrast. Officially, the PASS sign was abandoned on 17th September 1941 as it was clear that most units had no understanding of what the sign was actually for and were trying to use it as a priority pass; i.e. “I should be allowed past”, not “Pass me”. Once the AoS number was painted directly onto the vehicle the whole PASS sign became redundant anyway, as they didn’t have the reversible plate fitted anymore'.
