sounds like you got your answer, but i'll post my experience to add.
when i first got into doing any kind of metal work, i got a hand me down set (owned outright) from my dad that had been his dad's. they were only used occasionally in plumbing work. they were dated in the late 1960's, and hadn't been filled in years.
i took them to national, and was told if they had the correct valves (they didn't) they could still be filled upon an inspection. since they had some marking that they were originally purchased at a company national bought out, they swapped them for a used up to date set. they eve gave me a larger size free of charge since the size i had is no longer made. i just payed the refill fee.
btw, that's the only positive experience i've ever had with national. 
