Single Ended Check Nut Wrench w/ 15 degree angle.
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600 numbered wrenches are usually Check Nut wrenches. The same numbering system was used by multiple brands including Bonney, Fairmount, Williams , etc., although there can be variations between brands with some individual numbers. Check Nut wrenches are usually single open end wrenches, with a 15 degree offset, and a thinner jaw thickness, sometimes thinner than tappet wrenches.
Martin and Proto still manufacture a number of Check Nut wrench sizes in both SAE and Metric. Armstrong still manufactures a more limited number of sizes, but I think only in SAE. The Martin number for that Check Nut wrench is 605A. The letters are usually used to identify different sizes that may have been made from the same forged blank. "Special" is also sometimes stamped into the wrench to identify variations.