There is imac 27" (late 2015) core i5 3.2 Ghz. There are already two modules operatives 4GB.
Look at this option https://www.alser.kz/product/operativnaya_pamyat_c...
Like in all respects suitable, only the original maximum operating frequency of 1867 MHz, and then 1600.
Does anyone have experience installing RAM third party on your imac?