Hi First are you sure your faucet is Perlick brand? A big P at the top of the spout? Looking at the photo on Morebeer's site the stainless steel part between the O-rings is approximately 9.75 mm measured using calipers with 0.05 mm accuracy. Measuring the outside diameter of the O-ring is fraught with some error because determining if the blades are not compressing the ring but resting firmly against the ring is not easy. I measured this feature several times: about 10.35 mm is what I determined. I did not want to take an O-ring off afraid I might tear it. Measure the fitting's diameter within the groove of the O-ring and you should have a "good enough" measure of inside diameter. Measure the width of the groove and assume the thickness is slightly less. I cannot do it accurately without risking damage. Where to buy? My suggestion is to do a search to see if you can find that size of O-ring. Or track down the company that makes the growler fitting: Krome. The part number is on the filler. My eyes are way too old to read it. I had a hard time removing the filler from the faucet. I would recommend only removing right after use before the beer dries around the ring or after soaking in a cleaning solution. I use Kroger brand Oxygen Cleaner. Un scented Oxicleen is the same thing. Generic unscented is much cheaper.

This faucet is one no longer made. I recommend that you contact Perlick; they may be able to tell you or at least give you the internal dimensions of the spout that may help you find a suitable filler.

So. The 430 is a narrower outlet. The 425 is slightly wider. Look for another connection at more beer. I believe there are two versions. Choose the other one