All the "HJ" serialed steel 72s I have owned in the distant past were made by Norris industries, and the two I have left were both made in 1978. But they are not "78+" tanks but rather just plain old steel 72s.
I agree that the 83+ tanks are also going to be 3AA steel tanks where the "83" is the year of manufacture and the "+" indicates approval for a 10% overfill. They may or may not be steel 72s as I am not sure when production ceased, but that would have probably been just past the twilight of steel 72 production. Faber however made a very similar sized 2400 psi service pressure tank during that period of time that held 75.8 cu ft at 2640 psi. It was basically a 2400 psi version of the 2250 psi steel 72. If they are steel 72 sized tanks and are stamped "Faber" or "m303", or for that matter have a 2400 psi service pressure, that's what you probably have.
In terms of tank markings, "3AA" refers to the DOT approved 3AA chrome moly steel tank engineering standard and 3AL refers to the DOT approved engineering standard for aluminum tanks. The DOT used to be called the ICC, so early steel tanks (up to about 1966 or 67)may be stamped "ICC" rather than "DOT".
If you see something like DOT 3AA2250 12#78+ It would mean it is a DOT approved tank (DOT)made to the 3AA steel tank standard (3AA) with a 2250 psi service pressure (2250) manufactured and initailly tested in December of 1978 (12#78) and approved for a 10% over fill (+). The "#" will be one of several markings for the approved test facility that did the initial qualification testing of the tank. Subsequent requalifications (hydro tests) will have the same month/date format with the test symbol replaced by a 4 character RIN number in a 2 over 2 format like:
C2
00
denoting in this case "C200".
The E6498 (and SP6498) tanks were one of the special permit/exempt classifications that were lumped into the 3AL classification when it was standardized in 1982. They were supposed to be re-stamped 3AL when they come in for requalification (hydro testing). Sometimes they got skipped or were not taken in for hydro long enough that testers got out of the habit of restamping them.
The other tanks that were rolled into the 3AL classification in July 1982 were SP7042 tanks made by Walter Kidde as well as E8364 and E8422 tanks. A couple others would have been eligible - SP6688 (Norris Industries) and SP6576 (Kaiser Industry) - but those two companies never filled out the paperwork to renew the permits when they expired in 1979 and as a result those tanks are supposed to be condemned at requalification.
E6498 and SP6498 tanks were made by Luxfer from Alcan 6351-T6 alloy and are subject to the requirements for eddy current/ visual plus testing as part of the requalification process. Your LDS is totally wrong in thinking they are "clear" of the inspection requirements that apply to 6351-T6 alloy tanks. The "P" in an early Luxfer serial number indicates "80 cubic feet", but they are actually 77 cu ft at 3000 psi.
As for minor differences in the steel 72s, minor differences in dimension, shoulder, neck and bottom profiles as well as minor differences in weight were common. Steel 72s were made by several companies (PST, Norris Industry, Coyne, Taylor-Warton, etc) and variations occurred from company to company, from year to year and even from lot to lot.
As a general guideline a steel 72 will be more or less like this:
Service pressure: 2250 psi (2475 psi with the 10% overfill)
Height: 25.0"
Diameter: 6.8"
Weight: 29.5"
Buoyancy empty: 3.5 pounds
Buoyancy full: -2.0 pounds
Volume at 2250 psi = 64.7 cu ft
Volume at 2475 psi = 71.2 cu ft
In contrast, the current Worthington LP 77s will have:
Service pressure: 2400 psi (2640 psi with the 10% overfill)
Height: 23.2"
Diameter: 7.25"
Weight: 32.5"
Buoyancy empty: -1.0 pounds
Buoyancy full: -6.8 pounds
Volume at 2400 psi = 70.0 cu ft
Volume at 2640 psi = 77.0 cu ft
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Not to put too fine a point on it, but your LDS gave you a lot of bad information and is an idiot. Find another one.
