Dave G,
Regarding your question on the deck:
There should be a tag on the deck with an identification number. It should be on top of the deck on the left side as you're standing behind the tractor facing the front. I would guess it has a cast iron ended 38" deck since it's on a 100, but as you said, a previous owner could have cobbled anything on it. If you can identify the deck, it will make it a lot easier for anybody to help out. It may be covered in the manual for the tractor, or you may need to get a different manual for the deck.
Regarding your question on the deck:
There should be a tag on the deck with an identification number. It should be on top of the deck on the left side as you're standing behind the tractor facing the front. I would guess it has a cast iron ended 38" deck since it's on a 100, but as you said, a previous owner could have cobbled anything on it. If you can identify the deck, it will make it a lot easier for anybody to help out. It may be covered in the manual for the tractor, or you may need to get a different manual for the deck.