Yes Barometric damper, the metal swinging 'thingy' with the big knob.
Gas needs the double acting so it don't get blown out.
The MG-1 provides precise, accurate control of drafts at levels higher than permitted by a standard draft diverter, which is a frequent requirement with gas. Because it is double-acting, it opens out to relieve positive vent system pressures as low as .01".
In a duel fuel oil/gas or a gas-fired only application, use the M+MG2 as a double-acting draft control with the optional Field Thermal Switch accessory. Use the M+MG2 as a single-acting draft control for oil or solid fuel applications
The M employs side wings to control air direction with gate curvatures designed to compensate for differences in horizontal and vertical settings.
M is for oil and coal.
MG-1 for gas, double acting
M+MG2 everybody with a flue over 10" specify single or double acting
The M+MG2 is a switch hitter but needs to be specified for which team!