[Techy]

it appears the breaker bottom right is a 100A 2 pole, ... that should be your 'main breaker' just installed on the wrong side works on the same principal, but it looks like the left side has the hold down kit.

 

[Techy]

a plug on backfed main breaker requires a 'hold down kit', it looks like there is a 'hold down kit' installed in the position it calls out