You are correct. All three rings mount on a 74mm BCD. This is an "oddball" crank that Specialized used on MBs for a short time at the turn of the century. It didn't last long, and chainrings are no doubt scarce. Maybe an old time Spec dealer? Even if you could find a 42Tx74 ring it'll be a bugger to make it work. Chainrings for that crank were designed specifically to work together and other combos of rings won't without some creative spacing, bolts, etc. Time for a new crankset?