As Buff mentioned, HF... Not all that many 737s had these, mostly the ones that operated overseas, where the navaids (back in the 60s and 70s) were primitive in many areas of the world. (Yeah, like some things have changed.) This was all pre-GPS, and back when INS/Omega was pretty expensive, and not necessarily cost-effective for some narrow-bodied operators..
At Air Florida, a couple of our ex-Singapore 737-100s had them, and I think we later removed them. I have a vague recollection of one of the wires on one of our aircraft detaching from the tail, and subsequently beating the hell out of the fuselage, but I can recall any particulars...
While we are on the subject of asking what things are... can anyone explain to me the function of the sucker doors on the engines. I have seen them on early 737-100s and -200s as well as early 747s with the JT9Ds... though they didn't seem to have them for long.
Stop drop and roll will not save you in hell. --- seen on a church marque in rural Virginia