Retesting and filling oxygen tank.

I have had considerable difficulty getting FAA authorized oxygen service stations to retest and refill my 22 cu. ft. steel tank. It was originally purchased and installed in 1978 by Sailplane Repair Service in Ft. Collins, CO in accordance with a 337. Local stations here in Dallas, TX will not touch it without an assembly part number AND, even if it had one, they would only be able to service it if the P/N matched one for which they are authorized to service. My tank only shows the DOT 3AA1800 number stamped in the steel, but this is not the number required for the assembly. How do others in the U.S. get their tanks serviced, as I don't think the older tanks ever had such a number affixed? Do any owners of the newer aluminum tanks have similar problems with getting them serviced?