Hi Baz,
Yes, that is a Bison 4” Diameter 5C collet chuck. Bison even sells a 1-1/8”-12 BSW backplate for it.
**LINK**
I purchased mine from New England Brass & Tool. (USA)
This is a standard Bison item.
4” Bison 5C Collet Chuck (Plain Back) #7-862-0401 $495
Fully Machined Threaded Adapter 4” No.7-922-046 $95
Very happy with mine. The 4” diameter is the perfect size for the Myford lathe. Not available in a Set-Tru type.