Rectus is just a brand name and subsidiary of Parker Pneumatics.
rectus21 are a DN5 type 210 coupling, the pro 26 couplings that are on Gardiner are just DN7.2 type 260 couplings. if they are not stamped rectus they will more than likely be Mader, but it makes no difference as I have come across quite a few Madder couplings stamped rectus. not surprising as they are both German companies.the ez snap is a type 210 by the way
DN 7.8 type 250 couplings will fit the type 260
the type 21 couplings would not really be up to the frequent connect / disconnect of everyday use due mainly to there physical size but who is going to tell you this when you are giving them £7.00 a pop!
the 260 are a little bigger but you will find after some time of constant use they will leak.
one of the reasons is they only fit NBR o rings and they don't like any form of abuse. letting them dry out or getting grit on them soon wares them out. water is not to be seen as a lubricant and pure water as we know removes grease.
you could change the o ring for a vi-ton type these are more hardier and you should expect longer life. the main o rings in type 260 and 250 are usually 10mm bore x 3 mm wall, i know the nipples have a 12mm body this is to ensure a good seal.
please don't tell me you can not change the rings in these.
and yes i go through a few couplings in me other side of work, i have just ordered 50 type 260 safety couplings.
just in case you are going to ask, with safety couplings you pull the body back to disconnect flow yet the nipple is retained in the body, you push forward to release the nipple altogether.