Do I need an adaptor plate?
When installing a casement fastener using the standard keeper (as shown in the photo)- as opposed to the rectangular slotted keeper- there is a possibility that your job might be improved by the addition of an adaptor plate.
The casement fasteners are designed to work correctly on a pair of rebated windows with the assumption that the windows (sashes) are in one plane (flush) with each-other when closed.
Some older windows can warp over time, meaning that one sash will sit some distance forward from the other when closed. There is a tolerance of about 3mm built into the design of the casement fastener, but if there is a larger discrepancy than this the adaptor plate is available to be used as a packer behind either the keeper or the teardrop handle backing plate in order to bring them both closer to the same level, making operation more convenient.