Hi George,
I was able to check the MPC and the unique GT350 part number for the proportioning valve is different with C5ZX-2B091-A. I am assuming the "X" is for eXperimental and not Falcon. The MPC shows X for Falcon. The MPC also shows the GT350 valve is not serviced. Back in the day, this would required a GT350 owner to purchase the Mustang C5ZZ-2B091-B and have it adjusted.
When I had a proportional valve professionally rebuilt, I needed to specify GT350 and not Mustang.
Yours looks very nice.
Hope this helps
Cory
P.S. Please describe the date code on the metal plate cover with clip ring. Thanks