[WMichelsen]

Interesting how thick that square carriage retainer area is.
Another option is to add a washer. There is enough room between the spring and regulator.
Might have to file the corners down where it protrudes too tall.

I thought about a washer just to increase the contact area at that point.
When I cut and shortened my coupler (barrel nut), I installed it so the flat cut-off side was toward the regulator, for that reason. (The coupler itself is rounded a bit.)