I'm looking at Proof and specific gravity hydrometers from here http://www.coleparmer.com/Product/H_B_Durac_Plain_Form_Glass_Hydrometer_ASTM_125H_1_000_1_050_SG_0_0005/WU-08297-80
They conform to ASTM and are traceable to the NIST/A2LA manufacturing facility. Any reason why these wouldn't work?
Below is an expert from the TTB:
§ 30.24 Specific gravity hydrometers.
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(a) The specific gravity hydrometers furnished by proprietors to appropriate TTB officers shall conform to the standard specifications of the American Society for Testing and Materials (ASTM) for such instruments. Such specific gravity hydrometers shall be of a precision grade, standardization temperature 60 °/60 °F., and provided in the following ranges and subdivisions:
( B ) A certificate of accuracy prepared by the instrument manufacturer for the instrument shall be furnished to the appropriate TTB officer.