Thanks for the info!
I'm familiar with the mechanics of the hydrostatic test. Already consulted with the Marmic rep who did not anticipate a problem with my mid' 90's stamp dates.
From the link you provided: "most nitrous bottles whose DOT numbers begin with 3ALM are required to be tested every 5 years, with an unlimited lifespan."
So I should be OK there.

My local Airgas would only order me a 50 lb bottle of nitrous, and didn't seem to be interested in hydrotesting my existing tanks.
Anyway I'll post back once I get the test results