In message , Graham.
writes

Only seems to be worth £5 or so.
http://www.ebay.co.uk/itm/Meccano-A2...EASE-READ-/132
316976157

No, they are not worth a lot, but are good, solid performers and will
pass a modern PAT effortlessly. The one above is the A2, the next being
the slightly larger A3 which has a cutout that operates with a very
satisfying 'clunk' when shorted.

--
Graeme