Hi Tim,
I have just found that that this picture can be enlarged (click on picture) and yes indeed it does look like a free tenon. I also note that it has 2 sawn through peg holes on the end and so this could be a piece of old dry material used to make a repair to a damaged tenon. Some inside knowledge obtained directly from the Babcocks or the museum might help in this respect.
Jim, can you help here please ?
Regards
Ken Hume
