OK follow up to my previous post. I have been unsuccessful in all ways to read my SD card ID's via PC/laptop. However using Catymag's info on my NUVI 1490T I have been able to find the serial of my two cards, and using this in JetMouse I unlocked both cards for GB Discoverer without any hassle, and can see OS1:50,000 maps in both my 62S and 550T. The cards are interchangeable between the two units without issue.
I'll add my thanks now I'm here :-)
Ocicat