Need to identify Phono and AUX Board for NAC 72

[Aren]

Primemerkava, this was also a helpful thread

Nac 72 boards

I have a Nac 72 (internal image attached) I also have all the boards but have taken out the first four in the image. Can someone please explain what the first two boards spaces at the back of the unit as in image shown are for? I know boards three and four are for the phono/rca inputs but not sure what the first two boards do. Also I want to use an external phono stage with my Nac 72 Pre, what would be the best board configuration to use with an external phono stage using the phono/rca inputs…