In the spare OPO box in my office there is a spare retaining ring but for whatever reason no spare front plate PZT holder.

Since in the actual assembly the inner retaining ring only needs to be screwed down to the bottom of the wire extraction groove (see fig 1, 2, 3 of 2690), maybe I should just screw in the spare without replacing the front plate. I honestly have no idea why this PZT holder hole is even cut that deep.