I just saw this which may be relevant:
“ Look on the left side - there should be a matching one there too, as well as one on each side at the end of the end of the stays near the rear dropout (may have to look a bit for these). What I’m describing are vent holes, designed in so that during bladder molding all of the air that would be trapped during bladder expansion has an easy exit at what otherwise would be end of a dead-end tube.”