Borg Part #3101 - Fork Mount
This is the single-arm fork mount head. It may be used in alt-az or equatorial orientation with lightweight scopes such as the Borg Series 80 or Mini Borg scopes. It can be used with the Series 80 tube holder and adapter E set  (See below).
It has a 3/8"-16 hole for mounting on standard photographic tripods.
There are a few marks on the base from mounting to a Bogen Tripod (see pictures). Will sell for $185.
Borg Part #5503 - 80mm Plastic Tube Holder + Adapter E. Will sell for $35.
Will sell both as a set for $200.
Thanks for looking.