This is part of the chest strap of an Altura Morph backpack/pannier that I use for commuting. I've lost half the strap and one of my bikes can't take a pannier rack so I use backpack mode on that bike. That tends to be the bike I use in the rain; with a wet waterproof jacket the shoulder straps slide around in the absence of a functioning chest strap.
It's not a cheap bag, so I'd like to repair it
The part is question is plastic, a slotted tube with the dimensions shown in the first picture:
It should be prevented from easily coming off the end of the track, but some damage to that region of the left shoulder strap means the left half of the chest strap went missing.
If I can buy a replacement I can easily make a new chest strap from spares I have, but I've no idea what to search for. My guess is that it's generic but most of my other kit is quite old and I haven't seen one on anything else. I could make something or buy a generic replacement strap but this was quite a nice design that stayed in place unless it snagged on something.