I really can't or won't fault old flasks as far as usage. I will say that they are more prone to leak (breath) than new CVA gate valve type flasks. Older metal flasks are also more easily dented. For teaching, I use and older "sealed" flask and horn. For hunting, I use a more modern flask. One is a CVA tube with push-botton valve and the other is the TC clear view (plastic) again, with push-botton valve. If you like it, buy it, seal it and use it. If it's old and collectable, just keep it on the shelf .....
Be Safe !!!