Terry, I would say that is a positive stop for the maximum 10 gallon fill for your pump. With the seal on it, it makes me think that it was calibrated and bought off by weights and measures for whatever state it's from.
This is the drain back tube so when it is in the lowest position it keeps the cylinder from filling

You may be missing the other 9 fingers for setting the gallon level.

One other thing - 178's have a top with 4 attachment holes - some others have just 3


Jerry

DSCN6094.JPG $_59.jpg