Late 1931 CCPU Seat Riser Question

[Brian Fella]

Gents I have a late 1931 ccpu steel top with indented firewall. The seat riser was long gone when I got the cab and recently ordered a repo that is sold specifically for the late model cab. Not sure why it was different but anyway I cannot find anything in my books to show how far back it should be installed. In other words I am looking for a dimension from the back of the seat riser or front of the seat riser to something I can set to as shown in the pics attached. There are no mounting holes etc to gage off. Can anyone help me with a couple simple dimensions from their truck?