I think I'm missing a nut.

MoBros and MoSis'
I recently acquired a stock 225 engine from a 1974 Dart, automatic with factory air. When I went to put a breaker bar on the crankshaft to see if the engine would turn, I noticed that there was no nut in the center of the vibration damper. I then checked my other very stock 1974 Duster, and the slant six in this car has a large nut in the center of the damper. I also noticed that the cavity on the Dart engine where the nut should be is threaded for a nut, but this cavity is empty (see photos). Is there supposed to be a big nut there? Is there something different about an a/c car? I noticed that it has three pulleys on the crankshaft to accommodate the compressor. The outermost pulley appears to be screwed onto the middle pulley. I'll be converting this engine to non a/c, so is there any problem with removing the outer pulley? Any ideas about the missing nut? What size is it supposed to be. Thanks, all!

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