Unusual Patent Lump Neck Driver c1910

In 20 years of collecting and dealing I have never seen another one of these and it is not recorded in any of the classic text books on rare golf clubs. It is stamped W.Dobbie Patent St.Andrews. The back of the neck of the head is bulged. The head also has two plugs of lead. There is a small split in the head near the lead plugs but it does not detract from this rare club. Shaft and grip in good condition with usual wear.