Quoted from viperrwk:

In general, blue post Flash machines have Sys4 MPUs whereas the red post ones seemed to come with the Sys6 MPU, There are exceptions to this - probably around the transition time. I'm like you - I prefer the blue posts.

That's cool too. In general I'm a big fan of keeping things original but after upgrading the flippers the pops just didn't match the game for me. And it's easy enough to put the red caps and put back the original ones.
viperrwk