The idea for this very functional cover is that the arrow will indicate the direction of gas pressure in the pipes below. The arrow would be guided itself by a square notch on the rim of the seat. In this case there seems to have been some disagreement and a great deal of effort putting the round hole over the square notch.
The B&T of this cover is not the gas company, rather the foundry Bingham and Taylor in Culpeper Virginia. Among all the utilities, gas companies are the most secretive.
This size and type cover is found all over the place.