Joseph Lucas GEM Bicycle Lock

I ran across this vintage Joseph Lucas 'GEM' bicycle lock and wanted to ask if anyone has any background on it?  I know the lock was made in England based on the stamp but that is the extent of my knowledge on it. The key is missing so I am going to take to my local locksmith and see if they can key it.  Chain measures approx. 12" long. Lock is in good condition overall with some wear, rust, and nicks. Great addition to my Dutch bicycle!  If you have any info on this bicycle lock then let me know.  Thanks!