Alan Twinam says "Set 873/4/5 were London/Ramsgate workings with a mix of Maunsell Diner B.R Mk1 & Bulleid coaches.
And in that era would be a mix of Blood & Custard/Green" Here it is on a Victoria - Ramsgate working via Bexleyheath.

This train is diverted due to the same engineering diversions that will send 73081 to New Eltham on the following day. The headcode you will note is number 15 in this instance.