The 1C2 has a totalizer in it, that's what that extra plug is for. The 1D2 does not have a totalizer and thus no extra plug.
A 1D2 can simply be plugged in to the phone line and used unmodified.

If you put coins into it, they will stop at the coin hopper. If you want to tie the relay shut to divert coins to either the coin box or the return slot, there's instructions online. (easier to see it in pics than me try to describe the process)

Coins are not needed to make the phone work on standard phone lines, these phones used a special "coin line" supplied by the phone company to control cost of call, timing, and whether the phone kept your coin or gave it back.

For instance, if your call went unanswered on the other end, the phone company would send a 100 volt signal down the line to trip the coin relay and refund your coins. If the call completed, a 100 volt signal of opposite polarity would cause the relay to collect the coins.

Hope this helps!

Will G.