Those slots are for small arrows that on some versions protruded outside the skins and moved from left to right. These versions had embossed letters that said "To Car-To Storage" and shifted as you went from the gallon dials to the zero area. I have a Pipe Organ that has these arrows really adds to the pump.

Scott can you show us a picture.. Tom

Thanks Scott. Randy Ross contacted me and makes all the parts for that pump. I got the sign from him. It is porcelain and looks fantastic.
If nobody buys this thing I guess I will just restore it as soon as I have time.
Had a few people that said they wanted it, but nobody pulled the trigger.