DG1Z-12B577-A or -B. Lights and Siren Control Relay (Behind 2nd row seats.)

[MeadeCoSD]

So I've been looking for this, and I've noticed a huge difference in price between the older (-A) part # and the newer (-B) part #. The newer one says it fits 2016-2019 PIU's. The older one says it fits 2013-2019 PIU's. So why is the newer part so much cheaper, and why won't it (-B) work on the older Interceptors, if the older one will work on the newer Interceptors?? I'm confused!