5. Omega mounting plate. This is Omega’s 7.5”-wide circular metal base plate, which goes onto the large base board, and the enlarger is bolted on top of it. It’s for enlargers whose column has, at its bottom, a (smaller) circular plate that fits on top of this mounting plate
Check fleabay with a "Omega mounting plate" search. The mount you need is listed but in thrashed and trashed condition. Seller claims the hardware is included but no pictures are provided to confirm. Another is listed under "Omega base plate" but at 3X the price.