Hi
Reminds me of the 'Bardsley' boxes a well known NW builder used 38mm waste
in a chocolate brown .It was custom made for them deliberately (i am told)
to stop other plumbers doing repairs or nickin' it from site.
Believe Barrets also took up the idea.
I know it doesn't help much but as already posted a 40mm compression fitting
does seal onto it ,trouble is I have only seen white grey & black.
BTB if you are in NW I think Bardsleys still carry the stock .

CJ