A strange, cross-shaped tenon in the timber-framed Chester House in Knowle, West Midlands

Chester House in Knowle is also known as the Old Library. Part of the building dates back to the 15th century and the other half to the 16th century although, as one might expect, it has been remodelled many times over the years. It is free to visit and has an attractive knot garden behind.

Image from http://halmapr.com/news/apollo/2008/12/11/knowle-library/

Whilst walking past, I noticed this unusual tenon in part of the framing. I thought I'd put a picture up here as I haven't seen any others like it.

I believe that this part of the house dates to the 1400's and the tenon looks like it could be original to me. It seems quite unusual, perhaps just decorative or for some other specialised purpose?