It's not about fitting, but the first generation of FX Impacts is missing the left sided trigger roll support.
With those models the sear engagement cannot be adjusted in a safe manner because of lacking stability of the trigger roll.
The 2 sided support was added to the later models as an improvement.
My (ex-)Impact X did already have the 2 sided support, in that case it would have worked just fine. (My Impact X block with 10mm regulator was identical to the MK2 block)
Huma Air mentions it is designed to be used on the MK2 and M3 models, and that is done for a reason.
At last, I have built in these triggers in both my MK2 and M3, and in my opinion it is by far the best mod that can be done to an older model Impact.
The whole shooting experience got so much better, and finally the impact got the trigger it deserved in the first place.
Highly recommended!
Gijs
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