I haven't seen any code or standard references on any flat glass level gauge. The only reference you see is that the materials of construction conform to some standard. You will see some references to ASME and ANSI on the gagecocks.
I would give Tyco or other manufacturer a call with your concerns.
ASME B31.1 takes responsibility for level indicators in general. It is regarded as Boiler External Piping and Joint (BEP), chapter 122.1.6. Although I have to admit that the calculations in it don't really fit for glass gauges.