Question on Subscriber Plug/Jack Assembly

[Desert Phone Guy]

I recently picked up this jack and plug set.  The plug is marked 278A and the jack is marked 389A.  I had one of these years ago and I was told then that these were the predecessor of the 4-prong jack for residential service.  They are made of metal, the jack being steel, and the plug has a brass painted black housing.

I would like to know the timeline of when these were manufactured and used.  Photos are attached.

Thanks for any help.

[poplar1]

4-prong plugs and jacks were necessary for portable anti-sidetone sets such as 151AL and 202.
This 1932 BSP -- C36.201 issue 1 -- shows 3-, 4- and 8-conductor jacks and plugs: The 8-conductor was for the 205 multi-line set.

bsp-bell-system-practices -- c33-c38-dials-transmitters-receivers-cords/4597-c36-201-i1-feb32-jacks-plugs-inside-loc

(TCI Library)

[Haf]

Look here:
http://www.classicrotaryphones.com/forum/index.php?topic=20033.msg208875#msg208875

and here:

http://www.classicrotaryphones.com/forum/index.php?topic=20033.msg205390#msg205390

Haf

[Desert Phone Guy]

Wow!  Thanks for the great information, accurate and quick!! 

So, this being from the early 1930's, and having only 3 conductors, would it have been used with say a 51AL dial stick, or an early 302 desk set perhaps?

[Haf]

Everything not anti sidetone with an for e.g. 534 A  or 584 A sidetone subset where only three conductors needed.

Haf