I posted some links here somewhere for catalogs of reel tapes and parts where you can see the layout of the symbols and the reel strip number before you order them. The best way is to check the old tapes with the new ones to be sure the symbols are ALL match, but I know that is not possible sometimes. Disks can have have variations of symbols or symbol changes, operators could plug holes in the disks or punch extras, over lay or cut three symbols with a cut piece of reel tape to change a reel tape symbol.

The tapes have to correspond with the disks, so I check the numbers on the disks first, then check the combination of reel symbols to the disk numbers, then get the tapes. Mystery pay was a name given to the later model castle or blue front slot machine which Mills didn't have new style award cards made up yet for the ones they were selling, and which had a payout amount change, so they called it a mystery pay at first. Check you reel disk's (with the holes) and see if they are metal stamped SP-1 and SP-2 and SP-3.

Mills 4 reel slot machine