Air Venturi Washer/Oring question - Avenge-X PowerPlenum

need to put a new washer/o-ring for the PowerPlenum and it looks like there are 2 different types of brass/metal washers with rubber in the center hole. What are these things called and where can I find them? Looks like the ones i need are the thinner variant.

If anyone has experience with the avenge-x and the powerplenum and having trouble getting it to seal, please let me know how you solved this.

R
 
I ended up just ordering a DOWTY SEAL kit similar to this one for my BRK Ghost except mine was for bsp thread, and the avenge-x looks to use M8 thread.
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I ended up with TONs of sizes I don't need but trying to order the specific size I needed was going to cost more for one than I paid for the whole kit...

Like @Gerry52 mentioned a decent caliper will likely be useful.