Sorry I didn't get a chance to look last night. Looks like the info has been found! 16mm..... Hmmmm. One thing you MIGHT try. I have a right angle bit that has a 1/4in drive on one end (the other has the standard hex shape for those changeable bit drive screw drivers, etc). I've been able to use that to get in areas where EITHER a wrench (box or open) sits at a bit of the "wrong" angle or the nut/fastener I'm trying to get to is just a tad too far away... No ratcheting action, but you can "swing" it thru an arc to tighten/loosen stuff...

I finally was able to return to this project today. I am happy to report... success!

I used a 16 mm socket on a 3/8" drive ratchet with a 6" extension. I removed the group and accessed the nut through the hole there. The socket went onto the nut evenly and firmly. I then grasped the steam valve body with my free hand and got the nut as tight as I could while keeping the valve body level. I fired up the machine, and it still leaked from where the valve body meets the boiler.

I realized the problem really was that problem which afflicts these Gaggia/La Pavoni lever machines generally, namely keeping the base stable. So-- and I am not proud of this-- I put the machine on the floor, removed my shoe, stepped on the base of the machine with my besocked foot and repeated the nut tightening routine. Uing this method I was able to turn the nut about 1/4 turn more than before. I fired it up again, let it cycle a few times and... no more leak.

Thanks to everybody for their suggestions!

Jim