[JTinIN]

look at the picture on this post:
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The packs are not interchangeable within a housing.

You will need a navy type sear ready pack if you plan on changing the sear over to the navy pack.
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In the old days this would have been a good idea, except for the effort to take the sear in and out of the navy sear ready pack (which is a pain for most of us).

Now days the ATF "prefers" that you do not have any extra sear ready packs or they may consider them as machine gun conversions with out paper work (which is not a good thing ;-).

My preference is the SEF pack, however, if you would like the Navy pack, then would stick with that for both 9mm's.