Hi Crannogdiver, and welcome to the forums!
The Siebe Gorman Essgee Mistral was produced under license from Spirotechnique, it incorporated several changes and several generations over time - one of which was an HP port for a gauge!
Try getting in touch with Adair on the board for more info or check out his site CG-45.com. He’s got quite an extensive collection of double hose regs, including Siebe Gormans.
Be sure to post photos of your new acquisition!