Lost Gasket on Cartridge Filter

We have a cartridge module system that holds 4 cartridge filters. Each filter has a neoprene gasket on the bottom that usually comes out with the filter when you remove it for cleaning. However, last fall when we closed our pool, one of the gaskets fell down into the tube along the side, not down the center standpipe where it belongs. We did our best to remove all the water in the tube and looked for it, but have not been able to locate it.

Now that we're thinking about opening the pool for the season, we're wondering if this gasket is going to cause us a problem. Is it going to end up in our pump and damage it? Besides taking everything completely apart, are there any suggestions for what we might do? Obviously, we need to buy a new gasket for that one cartridge, especially because it's the one that has the pressure gauge at the top.

From what I can tell, this cartridge module is no longer being made, although parts are still available from Pentair.

Any ideas or suggestions are welcome.

Thanks!
Marie