Antique Vacuum Gauge.
This thing is so cool! Judging by the construction, we think this gauge dates to around 1900. The handle on the valve is wood that’s been stained black, (that’s one sign of an early valve), and the thread is an “oddball” non-standard thread. (another sign)
The valve itself measures 11″ across, and the outer rim is heavily patina’ed brass. Made in England by the C. Whittaker and Co. We could not find any info on this company. This thing would be the centerpiece of your Steampunk creation!