I think my confusion is because FZ4 moves from Erewhon to Alyina at some point but isn't a mobile factory (maybe), while the Grand Summoner is produced by ECG but is very much indicated to be terrestrial so I conflated the two.
Made me look up everything again in order of publication:
M&M (2009): Introduces EGC, stating they are made up of a bunch of mobile factories. Refers to them as initially being built and run "on" Ironhold. Not particularly conclusive since it's only a short section.
Objectives (2013): Places FZ4 on Erewhon. Also says ECG is in orbit around Erwwhon, but in the blurb under that says EGC is "Nominally based out of Erewhon, Eagle Craft Group is actually made up of a series of mobile facilities spread across the Occupation Zone." FZ5 was also on Erewhon but is destroyed / inoprative
Implies the factories are orbital, but could also mean that just the ECG head office is orbital.
TRO3145: Clans (8/2013): The Summoner entry states "The first Thor II to roll off the factory line on Erewhon, this ’Mech stands on a dais at the factory complex gate." And says it is produced by ECG. So heavily indicates at least some ECG factories are terrestrial.
Tamar Rising: FZ4 has now on Alyina, and is again definitely terrestrial. FZ5 is now on Blackjack. Are these the same ones or were the Erewhon complexes shut down and the identifications reused? Or Factory Zone 4 is a local designation so there are two.
I couldn't find any reference to Erewhon in the Hell's Horses novels.
So the first entry, to me, implies the factories are terrestrial, but the second is very clear in stating ECG is at least in some capacity, orbitally based. But then the Summoner II blurb says terrestrial again. And the moving of the very terrestrial FZ4 just confuses things (or at least me).
I'm starting to suspect FZ4 & 5 got moved because either someone thought the Alyina Mercantile League sounds better than the Erewhon Mercantile League, the map worked better that way, they made the same mistake I had, or it's just a straight up retcon lol.
As for ECG, if I had to reconcile this (without retcons) I would say there is a mix of orbital and terrestrial factories, with their HQ being an orbital one that is/was around Erewhon. Or the mobile factories have two components - an orbital part to make Endo and such, while final assembly is done on the ground. FZ4 is a second generation mobile factory - bigger and faster but less mobile, so it is its own thing but explains the movability.