Argon (shielding gas) regulator, specifically a single-stage cylinder regulator with a built-in flowmeter.
What it does:
• Reduces high-pressure Argon from a welding cylinder (typically 2000–3000+ psi when full) down to a safe, usable working pressure.
• The large gauge shows the cylinder pressure (the high-pressure side).
• The vertical glass tube with the blue knob is a flowmeter — it lets you set and visually monitor the gas flow rate (probably in liters per minute or cubic feet per hour, common for TIG/MIG welding).