But you should still be able to make it function properly as requested even if set to free play. On the Credit/Replay unit, there should be 2 (count 'em, Two) leaf switches that close when the reel is at anything but 0. One is for telling the replay button it can start a game (relay logic) and the other is to light the bulb in question. So, you can still close one without forcing the other one closed. Its just a matter of how you adjust the switch (or, if you use a jumper across two of the lugs).
Also, if you ever get a game with an original instruction pack, if you find a small foil type sticker (about the size of theh credit window in the back glass), it was to cover up the credits for the reasons described above.