Rear lamp

[Billy26F5]

Hi all,
In the early days Majors used the E27N rear lamp, then this one appeared

This one is slightly modified.
Some time near 1955-56, the socket was changed to the rubber type now readily available.
Anyone have any idea of the approximate dates of this change?
This earlier type lamp is now quite rare, although all Ebro tractors use it (the Ebro version is different).
Sandy

[Billy26F5]

By the way, anyone know where there's a plug for Billy's socket? (as in the pic in the previous post)
Sandy