Here's a few pics of the L850, with the working and over filed key.



The lock that arrived today has been great fun. I tried, in vain, to SPP it with a Southord hook. It was full of old grime and muck, so much so, that for a few minutes, pin 1 seized after being pushed down. I flooded the plug with Wd40 and jiggled around for a while and thankfully it jumped back up again. After 10 minutes of having apparently set all the pins, I gave the plug a hard push with the tension wrench and it shifted by about 15 degrees, due to being so dirty. Lots of Wd40 later I got it to the stage where it was giving the endorphin giving pop when picked.
After taking your advice about impressioning, I mapped out the approximate pin heights, with the cylinder held in the open position by a tension wrench with a rubber band wrapped around the body. Just by chance I happened to have a Yale key on a 25 year old bunch of keys sitting on the computer table. This key looked unbelievably like the pin heights I had just felt, apart from pin 5. A bit of solder later and 15 minutes of impressioning, I couldn't believe it when the lock popped like the key was made for it, amazing. Will post a pic tomorrow. Just shows it's always worth a go. So that's two 50+ year old Yales that are now back at work.
Thanks so much for your tips and advice, it's great when these things work just like they should.
all the best, Nick.