Item Menu BG1 Layer Positioning

I'm not sure if I understand correctly the question but top BG1 write row is fetched from $49 (see $C383F7) then stored in $E6 and used in draw inventory (see $C37F88). Make a search for $49 and instead of STZ $49 (init at row 1) store #$01 in $49 (see $C31AE2). I hope this somewhat help..

Edit: There's also the BG1 V-Shift table at $C37EE7 but while the data is likely positioning I don't know its format.