Reflex Sight Glass Level Gauge - aka a Gauge Glass
Sight Glass Level Gauge Indicator Boiler Level Sight High Pressure High Temperature Boiler Hydraulic Tank Level Control Boiler - Instrument Parts & Accessories - AliExpress
Sight Glass Level Gage -Oil, Hot Water Tank Reflex Reflective Glass Level Gauge - China Reflex Level Gauge and Reflective Level Gauge
WIKA - Level indication with sight glass indicator - YouTube