After all I had to settle on some 70000 series food grade nickel plated brass push-in fittings made by AIGNEP for 4mm tube, 1/8 BSPP thread size. The latest catalog I got from AIGNEP lists them as 70020 00 002 and 20 bar rating but the ones I got are apparently rated to 15 bar per their eBay description, which may be a previous design. 15 bars rating vs. tired 15 bar pump, seems acceptable at least on paper, I was skeptical but they seem to hold the pressure just fine in my limited tests using very fine grind and double shot filter/button, no leaks, which left me cautiously optimistic. I had to cut the tubing and discard the brass stop ring/insert, make sure you cut as close as possible as the tubing doesn't have much length to work with.
Last edited by Javaphile; 6 Days Ago at 10:27 PM. Reason: eBay link removed