I don't recall that fitting in my 1974 144, but then I never had the tank out. I did some reading and it's as Derek says, for a fuel return line.

In the earlier D-Jet systems it was apparently a double barbed Tee fitting for a return fuel line from the CIS pressure regulator and some kind of overpressure line from the pump. If your '72 142e ('e' for einspritz, German for injection) is K-Jet (I forget the switch over, either '72 or '73) then they apparently eliminated the fuel pump overflow, now just using a barbed brass elbow for the return line.

Hopefully you can find the return line left hanging somewhere with the brass fitting attached. If not then the shop probably needed to take the fitting off for handling and neglected to put it back.

I see a comment not to force it too much as the fitting on the tank is soldered and might crack, then needing re-soldering. I'd say inspect the solder carefully for fractures before installing the tank as you may not otherwise know until you eventually smell gas vapours drifting up into the rear wheel wells, but having been at a good shop they hopefully checked it carefully. If in doubt, with the tank off you could plug the fitting or put a small length of hose on it with the open end held high, put a bit of gas in the tank and tip it up with that corner down low to check for any hint of a leak, maybe leave it overnight (not in the house) with rags tightly stuffed into the filler tube and pickup assembly holes.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now