From The Anvil  Monkey tail fastener with mortice plate 33142

Window Fastener Monkey tail Beeswax 33142

Not handed these are reversible as the handle flips over.

4.5 inches long Beeswax finish
Fixing Plate 2.375 inches  x 0.75 inches
Mortice Plate 3 inches x 0.75 inches

It is also suitable for the use on windows that have a draught seal.
For windows without a central mullion the hook plate 33237 may be required and can be ordered separately.