Just for the record, a mouthpiece is not ’One-Size-Fits-All’, the regulator orifice size varies on some reg’s. Here’s a blurb from Dive Gear Express listing the different sizes:
The only difference is the orifice which mounts on the regulator
mouthpiece fitting. You will need to select the appropriate size for your regulator. Almost all regulator
mouthpiece fittings fall into two general sizes with some fittings being only slightly larger or smaller. The
slightly smaller fitting retention lip (about { 1&1/4 in x 3/4 in |
3.2 cm x 1.9 cm }) takes a SeaCure 1, and the slightly larger fitting
retention lip (about ( 1&7/16 in x 7/8 in | 3.7 cm x 2.2 cm })
takes a SeaCure 2. There are also the Seacure 3/4/5 specialty sizes
which fit the unique retention lips of some specific brands including
APEKS and Poseidon.