Hi PP, Usually called a shroud (stay) plate, -or a Tang if it is a strip type riveted to the outside of the mast (usually on the sides for Shrouds). The ones with wrap around 'fingers' riveted on to the outside of the mast are shroud Yokes usually for hanging halyard pulleys from for hoisting jibs etc (the eye faces forward). The double strip type with the clevis is another type of riveted tang which faces forward or is mounted on the side. Pic of a big yoke on a mast, (yours is probably a fitting just glued into your mast to represent the one in the pic). Any of these fittings could be made easily from thin brass sheet.