It is still common for certain items to have postage guaranteed for return but I'm not sure the post office will deliver such things anymore. The phone number is for AGS HQ, which we know from the matchbooks I have shown here previously was at 1619 Bedford Avenue, Brooklyn, a mere block-and-a-half from the rightfield wall at Ebbets Field. I have to scour some old ballpark photos to see if the HQ shows up at all.
It's actually a charge coin, which if you click the charge plate link above, basically dates to just after the end of the Civil War. The truck was called their "Trouble Shooter" by the way. If that is a running count of AGS charge customers, it's impressive!
I tried to decipher the writing on the truck's doors and side panels but I think it's just gibberish, although some neatly executed lettering is evident on one side:
The other side's lettering is similar I think but it's a little rougher than the other view:
I used a high power magnifying glass but cannot resolve anything lettering-wise here either. Bummer.....