I still have rope/wire halyards - which sawed through my old restrainer - some weird galvanised bracket I needed to replace anyway!
Personally I installed this:
http://www.westmarine.com/buy/schaefer--halyard-restrainer-bracket--285538Been there about 4 years no, no signs of wear, works as advertised. I drilled/tapped the 4 boltholes, installed it per instructions for my roller furler - a bit higher than the original position. I opted for the Schaefer as no moving parts - I am deeply suspicious of sheaves :-) but I am sure the Harken equivalent would be fine, they make good stuff.
I don't recall a similar tang on my mast; the mast cap for me is held on with 6 (?) crosshead screws an inch or so down from the top, on either side of the mast (not front/back). I am sure folks who have had their mast down more recently can provide more accurate information. Probably obvious, but some owners install an inner forestay for storm sails...could the tang be a vestige of such a setup?
Good luck!