Here is the canopy in the closed position, demonstrating the tight fit between the sliding hood and the fixed windscreen frame.
A notable detail is the small red peg, perhaps some more knowledgeable readers could help with explaining its function.
Guided Tour of the Spitfire PR Mk XIX
The Lobelle Canopy
Here is the canopy in the closed position, demonstrating the tight fit between the sliding hood and the fixed windscreen frame. A notable detail ...
From comparison with the windscreen/canopy of the fighter versions it is almost certainly the ‘ normal ‘ canopy release, to allow the pilot to open the canopy from outside when boarding.