You don't need a "kit" to service those old valves.  In the
70s valves were made using off the shelf o-rings.  As I
remember, the stem o-ring was an #010 and was 70-75
durometer.  Simply take your old o-rings to an o-ring
supplier and just match the hardness and size.  Of course,
you will need to buy the proper relief (burst kit) from a
dive shop or dive parts supplier. They usually run about $10
or less. Total parts shouldn't run you more than about $12
per valve.

Just my $.02