What is this fitting?

[Callerton]

Help, please.
About 10 years ago kitchen fitters used these push fit elbow joints to connect 15mm pipes under the sink. One failed & I had to get out an emergency plumber.
I've been doing some research about push-fit connectors & it seems they have different means of disconnection. But I can't find anything on line that looks like these. Especially as it seems to have some sort of external sleeve sticking out from the fitting.

Does anyone have any suggestions please? e.g. make, how to connect / disconnect. I'm particularly interested in how to disconnect (preferably without damaging the pipes) so that I can repair myself if/when they also fail. From what I've read the JG Speedfit system is probably the best replacement.

[Stevemr]

Hi. Pretty sure they are old style Hep2o fittings.
You simply unscrew the end cap. Then the pipe will come away from the fitting leaving the end cap rubber o ring and metal grip thing on the pipe.

[ARH]

But it probably won't come off if there is still pressure in the pipe, so turn off the water (which to be fair is obvious) then turn the tap on.

[Callerton]

Great.
Many thanks.