Not that it helps but the DOT published a list of tanks made from 6351-T6 aluminum.
The Department of Transportation has released the
following list of scuba tanks that are most likely to be
made from the 6351-T6 aluminum alloy to which a
disproportionate number of tank explosions are
attributable:
All DOT3AL tanks manufactured under one of the following exemptions or
special permits: 6498, 7042, 8107, 8364, 8422;
All composite cylinders manufactured under one of the following exemptions:
7235, 8023, 8115;
All Walter Kidde DOT3AL scuba tanks;
Cliff Impact DOT3AL scuba tanks made before July, 1990;
Luxfer 80.8 cu. ft. scuba tanks (S80.8) made before May, 1987;
Luxfer 72 and 100 cu. ft. scuba tanks (S72, S100) made before August, 1987;
Luxfer 80 cu. ft. scuba tanks (S80) made before January, 1988;
Luxfer 50 and 92 cu. ft. scuba tanks (S50, S92) made before April, 1988;
Luxfer 30 and 63 cu. ft. scuba tanks (S30, S63) made before May, 1988;
Luxfer 40 cu. ft. scuba tanks (S40) made before June, 1988;
All other scuba tanks made in the U.S. before February; 1990 (except Catalina);
All scuba tanks not made in the U.S.
Per the DOT list, a Luxfer 30 made in May of 1988 or later would be made from 6061-T6 alloy and would not be suceptible to sustained load cracking.
Personally, I tend to avoid shops with policies of not filling properly inspected tanks given that there has been no incidence of a properly inspected 6351-T6 aluminum alloy tank exploding since the implementation of eddy current inspections over 8 years ago. Filling one is not a safety hazard and shops that suggest it is are either ignorant or are perhaps trying to promote tank sales by telling you your tank is unsafe. Either one woudl be reason enough for me to take my business else where.