There seems to be a bit of confusion...
Centhot: You must mean the Spitfire canopy. My notes above were for the Hurricane. The Hurricane canopy did not change throughout production.
However the Spitfire mark numbers are somewhat similar. The Spitfire Mk I began with the same/similar Merlin II engine and a 2 bladed prop. Later upgraded to the Merlin III and 3 bladed props. The hood was upgraded from a flat top and flat sides to curved top and flat sides, but that mod had nothing to do with the Mk numbering.
Mk numbering is based on the engine. The suffix a, b, c, d, etc; is generally descriptive of the armament carried, but not always as it was not standardized for all aircraft.
By the Battle of Britain, all Spitfires, but exceptions probably existed, were upgraded to Mk I (late) standards: Merlin III, 3 bladed prop, armored windscreen, seat armor, improved canopy and radio. Most of the external things we modelers see. It was still a Mk I and not a Mk Ia.
When the cannon were tested/added in Fall 1940 to the Mk I it was denoted as a Mk Ib, and then the earlier, 8 gunned Mk I was denoted as the Mk Ia.
The Mk II had the Merlin XII (external difference is the Coffman starter bulge on the right nose cowl) but otherwise the Mk II was the same as the Mk I: IIa had 8x.303; IIb had 4x.303 + 2x20mm. There was a Mk II LR as well that was a IIa with a long-range tank added to one wing (not jettisonable.)
The Mk III, a prototype only, 2 made, used the Merlin XX (same as the Hurricane II). But because of priorities the Hurricane got the Merlin XX production and the Mk V resulted using the Merlin 45 series.
Hope this helps.
Tim