Hey there,
Thanks for writing in! Your casement window lock looks a lot like our 38-097 LH EntryGard Sash Lock. The right handed version of this lock is part 38-098.
In regard to the "back piece" are you referring to the hardware in the first photo? If so, this is referred to as keeper. The keeper compatible with these locks is the 38-079 Casement Sash Lock Keeper.
To confirm that these are the replacement locks and keeper you need, carefully review and compare the dimensions below.
I'm happy to help! If the window doesn't have any hardware on it, you'll likely need to purchase both the lock and the keeper. Both parts are sold separately.
The keeper is what helps to secure the latch inside the casement lock to keep the window locked. So it would be necessary to have the.
The handing for this lock is determined by the side the hinge is on when viewed from the outside. Also, you'll notice on the left handed lock you provided the photo of, the lever is pointing downward. I hope this helps!