I have a Bogen/Manfrotto 3046 Tripod with a 501 head. This head is an early, smaller version of the 501HDV head. You can find them on eBay at much less than new. With patience you can get something like this for under $200.

These use a shoe that fits with a 1/4-20 threaded screw onto the binocular bracket. The shoe attaches to the head. Shoes can be unique to onc exact head, or a shoe can be campatability with several heads.

When buying used, be aware that if you get a head without the shoe, you will need to buy a shoe. As I found out the first time I bought a used tripod, new shoes can cost $15,25, or $35. Then it's a matter of paying full price for a new shoe or waiting days, weeks, months, years to find the shoe you need on eBay or here for $5 or $10. It's a yellow flag or a red flag if the seller can't or won't tell you if they are including a shoe with the head.

SL