I took a look at what we have, and I was able to find the:
-Gen-Y 2-Ball Mount for 2-1/2" Hitch Receivers # 325-GH-064.
This ball mount will use the larger 3/4" hitch pin # 58388 that you mentioned you have, however this ball mount is designed to fit into 3" x 3" trailer hitch receivers.
If your receiver opening is smaller, you can use the Curt Hitch Receiver Reducer Sleeves # C45408. This reducer sleeve will allow you to adapt the ball mount to your 2-1/2" or 2" receiver opening.
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