Billy Anorak and Steamport Southport, I love the pictures, thanks ever so much for posting them. Whilst on the subject of liveries I'm sure you'll already know that as well as BR blue with full yellow ends, there was for a period BR blue, small yellow panel. I've only seen about 3 phots of the SYP jobbies, only one of which is in colour in the splendid book 'Merseyside Electrics' by Jonathan Cadwallader & Martin Jenkins (Ian Allan, ISBN 978-0-7110-3417-4, Published 2010). The photo is by A.F. Graham and shows a 503 at Wallasey Grove Road in Feb 71 and the caption states that the yellow panel first appeared on Wirral EMUs in 1967. So it seems that at least one of them lasted a few years like that and also raises the possibility that 502s were similarly treated.
Jonesy