I'm not a fan of most rubber bits like this...they are thicker and sadly all I had contact with broke-down from the enzymes of the horses saliva... No thanks to rubber chemical components swallowed.
Why would you look for rubber mouthed and not a finished exterior of metals in various compounds?
Some of those metal compounds do encourage salivation and some do not but few are known to dry a mouth like hard rubber bits can.
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These are other options very similar easily found in a google search.
From various metal compounds to create the bit, ergonomics for fitting the bit in the oral cavity, to choosing how you want that lozenge to lay when the bit is engaged....
To me, the heavy of the rubber and not sleeking of what must be carried in a very small groove opening...
There is a reason why so many bits are made in different thickness configurations....proper fit.
I do not like O-rings for the chance of catching a lip edge. I always feel I have to up the size to protect the lips...that can throw off the fit and communication, but that's me.
🐴....
jmo...