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Locally from On-Target.co.nz.
NZAR15.com also used to sell them, only seems to have Gen 3 now.

Rwt the middle shortie 11.5" build has the Mid West SS Gen 1 free float rail. That one is 9".
Wanted the slimmer Gen 2, but had to get the Gen 1 to have clearance over the JP adjustable gas block....

Or do what I do, have a separate spare A-Cat thumbhole stocked lower receiver for my E-Cat rifles, swap the lower out and take off the flash hider, use 7 shot mags and its a complete A-Cat rifle.

Sounds all good.
Yes, you can use a carbine or rifle buffer/stock on a mid length AR. Carbine buffer gives you more options for an adjustable stock obviously. The adjustable rifle stocks are pricey....