I think it's likely that a common blind was used for 313s and 317s (when they used to have blinds. This is the whole list:
(White blank)
(Red blank) [third tail-light for Northern City Line]
Audley End
Baldock
Bishops Stortford
Broxbourne
Cambridge
Cheshunt
Chingford
Depot
Ely
Enfield
Finsbury Park
Gordon Hill
Harlow Mill
Harlow Town
Hertford East
Hertford North
Hitchin
Hitchin via Hertford North
Huntingdon
Huntingdon via Hertford North
Kings Cross
Kings Cross via Hertford
Kings Lynn
Letchworth
Letchworth via Hertford North
Liverpool Street
Moorgate
Moorgate via Hertford North
Peterborough
Royston
Seven Sisters
Sorry Not In Service
Spalding
Special
Stanstead Airport
Stevenage
Stevenage via Hertford North
Stratford
Tottenham Hale
Ware
Welwyn Garden City
(Red blank)
(White blank)
It's a real annoyance if another driver arrives at Hertford North and winds the blind up to the red next to Audley End upon arrival, as if you then take over, once you arrive at Moorgate you have to wind through the whole list to get to Welwyn! The red blank next to Welwyn becomes faded because it gets used the most, as most drivers use this red blank because it's more convenient.