I'm not sure to be honest. If there was a bar for other electric locks behind the frame it would have been quite simple. It would involve the casting off of one wire from the incoming link to the block instrument and it's replacement with a new wire from the block to a biassed polar relay (BCR) and another wire from that relay to the incoming link. After that several new wires and another relay (BSR) would be needed (1000 ohm shelf type) a two way circuit controller and an electric lock and separate economizer. The ironwork would be a good days work and the wiring another.
http://richard2890.fotopic.net/p56209378.html
I'll have a trawl through my paperwork to see if i can pin down a date when it was fitted.
There are still boxes with free starters out there.
On the subject of ironwork... a little bit of a nearby box is going back into use...
http://richard2890.fotopic.net/p64539712.html
The locking trays are from Hitchin Yard box.
Sandy s/box.
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