I can't speak for the Echlin part but it cross references with BDW part R218 available at Advance Auto. I found this thread trying to track down a horn relay for my 71 Challenger.

It is a very close match to the original relay and DOES include the key in buzzer.

The relay fits in my fuse box and works with no modifications. But there are 2 slight differences which could easily be fixed to make it a closer match. The bracket, on the BWD part at least, is held on with what looks like 3 small peened over mounting studs, which could be easily ground off. And three of the blades (the three in a group on one side) have slight steps at the base making them a tad too wide for it to fit in the fuse box flush, again easily fixed with a dremel.

But again the part fits (mostly) and works in my 71 Challenger as is. Thanks for this thread it helped get my horns going again after 9 years, since some jackass at a shop removed mine to kill the key in buzzer, and then proceeded to loose it.