The Ma37 is not a prototype it is far older.
The Ma37 (the weapon you are attempting to skin) is a far inferior weapon to the Ma5b (the model you are skinning to look like the Ma37).
While the Ma5b has been in operation in the Marine corps since 2504 the Ma37 has been distributed across all branches since 2437.
The reason Ma37s were seen on Reach is because they were issued to the Army. Since the army is non-mobile it is primarily a holding force and rarely has to deal with anything tougher than insurrectionists.
With this in mind the soldiers on Reach would be the last to receive the less effective Ma5bs
The Autumn actually had stocks of Battle Rifles as one is present in Halo: First Strike delivered by Johnson's pelican.
However MC never reaches an armoury or is knowingly deployed in combat where such weaponry would be useful therein never bringing it from stock.
(The real reason is quite simply it's an old game and they hadn't created the weapon yet)
And I dearly hope that spoiler isn't true. Getting my copy of reach on release day and will be unhappy if something as cheesy as that is done :(
For a start we already know the Autumn was involved in the defence of Reach. Such a result would be quite disappointing tbh