What makes you think the MoM Y is a pyro? I don´t think so... It looks just as well made as the other heros, and there´s some pics which show it with the same parts as the other heros which now differ, even one more part on top of the original Tyrell engine part on the back. It´s got several mounting points, had lighting... Or were the ANH pyros built equally well?
Darth, I'm basing my guess on the following sources (although there is some conflicting information):
According to Lorne Peterson in “Sculpting A Galaxy” (2006), only
two (heroes) were originally built. That book also mentions that one hero was given to 20th Century Fox Executive Alan Ladd Jr. as a gift, and that, at the time of filming ROTJ,
no hero Y-Wings were available,
only an unexploded “pyro” which was seen at the end of “Empire Strikes Back”.
According to Star Wars dot com databank, three Y-wings were built, but they don't specify prototype/hero/pyro.
A third source (the Internet Movie Data Base) reports that the "
remaining two Y-Wings were deemed unsuitable for use by the time ROTJ was to be filmed. ILM needed a new Y-Wing hero model, so they decided to use a home-made model built by Bill George before he even joined ILM."
I'm pretty sure
Gold Leader (Dutch) and
Gold Two (Tyree) were the two heroes that Peterson mentioned, and both are now in private collections.
That leaves "
Red Jammer" (considered a prototype and not a hero filmining model), and "
Gold 3" (the MOM Y-Wing).
Gold 5 (Pops) was apparently destroyed during the filming of ANH.
Remember, some of those ANH pyros looked pretty good (have you seen pictures of the Red 12 X-Wing?) And
Gold 5 looked very detailed from far away, even though it was a pyro.
I'm convinced that the MOM Y-wing is the one that Peterson mentioned, and I think I spotted it in ESB just like he wrote. In that scene,
it even had its engine lights "on"! (pyros normally don't have lighting effects)
As for why it looks more detailed than expected for a pyro, I have no idea. I agree that offhand it doesn't look like a pyro. I can tell you that members on this forum have discovered pyros with hero-like features (see old X-Wing posts). Maybe they "re-worked" it for ESB?
Summary: (original ANH Y-Wing models only; just my theory)
Red Jammer - prototype - never filmed
Gold Leader - [hero model] used in ANH & never seen again (now in private collection as per SWPropmaker)
Gold Two - [hero model] used in ANH & never seen again (presumably this was the one given to Alan Ladd Jr.)
Gold Three - reported to be a pyro, possibly reworked & used in ESB only, became the
MoM model
Gold Five - [pyro model] - destroyed in ANH & never seen again
Plus at least two other pyros built for ANH (tp portray the deaths of Dutch & Tyree), and four more for ROTJ
Note: some of the "missing" models have been seen in many books, but there is no way to determine when those pictures were taken - the books don't say.