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This unsigned colonial semi circumferentor was featured in the 1964 Bedini book (PDF) at Figure 34 and the accompanying text. As indicated in the book, Bedini owned this instrument, and I came across a number of pics of the instrument while I reviewed Bedini's files at the Smithsonian. I included a front and back scans of one oft he Smithsonian photos below.

In his book, Bedini describes his semi circumferentor:

"Of equal interest is a large semicircumferentor made by an unknown American instrument maker in the second half of the 18th century. The instrument (fig. 34) consists of a plate of hammered brass attached to a quarter circle block of mahogany, with a glass covered trough compass within a silvered opening, and the gradations stamped into the brass. The brass sighting bars are attached to a swivelling bar that can be fixed in place with a set screw underneath the block. The instrument, which is in the collection of the writer, is not signed with a maker's name. Its workmanship is excellent, and professional."

The Semi Circumferentor lacked sight vanes and thumbscrews when owned by Bedini. A subsequent owner had Jeff Lock make replacement vanes and thumbscrews - Jeff did a wonderful job.

Here are the specs:

14 inch alidade

8.5 inch needle

Inner scale to 1/2 degrees

This is a great opportunity to own Bedini's colonial semi circumferentor.