I looked into this a little more and TT is correct. Tektro's 300 series should all have a 39-49mm (short) reach, while the 500 series should all be 47-57mm. The one I have came from BikeIsland's "economy" brake set and is definitely short reach witha recessed nut, so I'm sure they don't even know what they're selling (which doesn't really surprise me).
We've also got a 539 front caliper sitting around and it's definitely medium reach.