Jim, don't know if it would apply on an M-17 or not, or exactly what you have in mind. But on my first M-15 #264, I sheet-metal-screwed strips of aluminum scrap to each side which had the same effect as continuing the slot downward. On my current M-15 #361 I drilled some 9/64" holes through the mast that intersected the slot, then use a 1/8" S/S cotter key extending through the holes to provide a stop for the boom. (Actually, several sets of holes to provide different "stops" for the boom.) This mast had the slot made just by bending the lips outward to make the opening (whereas the older model had an actual cutout that had to be bridged) so I just carefully hammered the lops back into place. Doesn't look perfect ( don't either) but doesn't yell at you. John Butler - Rejoyce! is # 361, Joy was #264