I am trying to ID the head that is currently on my B7100HST.
The Casting marks are from back to front K5-B 04 03 with a 7 above them. The head also has a coolant access port next to the front injector. see picture.
The Kubota cylinder head you have is a Kubota D750. This site has a Kubota D750 cylinder head for sale that will fit your B7100 hydrostat tractor. Kubota Engines, Kubota Motors, and Kubota Cylinder Heads I got a cylinder head from them for my L175, works just fine. Hope that helps you.