my guess is that when they took away the old filter and added the big sand filter, they used a couple of 90's to get to the heater and back to the pool.

i replaced the cartridge filter after a muddy water rain deal with our new construction...... straight into the new pool.
it was a pain to clean those cartridges, so thus the change. i'll pick up a new gauge, as i think this is the old one from the old filter.

thanks for the input.