When I purchased my '77 4 1/2 yrs. ago, one of the tanks had an OPD valve on it and the other didn't. I remember several dates stamped on the collar of the OPD valved tank. After a couple of years I took the other tank in and had an OPD valve put on it. The Suburban Propane guy inspected it and I think a new date was stamped on the collar best I can remember.
'01 2500hd ext. cab, 8.1 litre gas, 5 sp. Allison auto
3.73 rear end
Mag-Hytec rear diff cover
Amsoil Dual by-pass oil filtration system
Amsoil synthetics all around
265 watt AM Solar, Inc. system