Reporting back now that I got the new stainless mushroom from CCS and have it working in my Rocket Giotto V2.
I'm told it's the same mushroom that you'd get in some of the newest Izzo machines.
It was shipped complete with a stainless gicleur and gicleur screen, but without a top washer or a lower o-ring. It took the same size as a spare I had, but it's probably a good idea to buy a new o-ring when purchasing this mushroom. (Edit addition - Do buy a new o-ring when purchasing this mushroom! The thicker o-rings (like the green Rocket o-ring) may be too big.)
Here's what it looks like next to the standard plated brass rocket mushroom:
Dimensions for the length of the lower sleeve, o-ring groove size
**, depth of sleeve to spring seat, filter screen size are identical. The new one, being of stainless, is a little lighter (246g rather than 270g.) Note that the bottom sleeve is of a piece with the upper part, just like the old one - it doesn't unscrew like the ECM one (see below for a pic of that one ) . The cap takes the same 36mm wrench as the large nut of the old mushroom
** Edit: I thought it was the same, but when I tried again using my thick green Rocket o-ring instead of one from a rebuild kit I had difficulty getting the mushroom threaded into the group. So I remeasured carefully and the diameter where the oring sits is about 21.5 in the new mushroom but was only 20.4 mm in my Evo V2. I checked my older ECM Giotto, and it was 21.5, so I think the Evo V2 (that came with the green o-ring) may be the odd man out here. I think this is an area where E61 is not perfectly generic -- note that Stefano includes both a thin and thick o-ring in his E61 rebuild kits.
The gicleur is stainless and is removed using the same 7mm socket or nut driver that is used to remove the old one. It also has the same M6 threading and the two can be easily interchanged.
I believe the new one is 0.6mm. Below for reference is a picture of my stock Evo V2 gicleur ( 0.7 or 0.8 mm I'm told ) at the top, the new stainless gicleur, and an aftermarket 0.5 mm gicleur at the bottom.
I did a quick water debit test of the stock vs the new gicleur and the flow dropped slightly. (230 ml in 15 seconds for the old one, 185 ml in 15 seconds for the new stainless one. I decided to go ahead and use the new one.
A big difference with this is that to remove the mushroom from the group you first unscrew the cap with the 36mm wrench, and one of two things can happen. Ideally the mushroom unscrews along with the cap, but the cap might unscrew from the mushroom body. If that happens you'll need a monster 14mm hex (allen) wrench to get the mushroom out. Here's what the top with removed cap looks like:
The cap has a recess for the gicleur screen, and has a ridge that seals against a teflon washer that's in a recess at the top of the mushroom. That teflon washer is one part that you won't get in a standard E61 rebuild kit.
To tighten the cap against the mushroom body you just put a 16mm wrench on the flats near the top of the mushroom sleeve and crank the 36mm capnut tight. I think if you get this good and tight you have a better chance of the mushroom coming out with the capnut when servicing the group.
Note: The ECM stainless mushroom is different. The bottom sleeve threads into the top body as seen in this picture: