It's an S2 judging by the side windows. S1 side windows were 1 piece and removable. S2 are 2 piece with the main portion electric lift.
P.S. This would be a huge undertaking to restore...
Series 1 side windows are indeed one piece but not removable. I have seen several mods to improve airflow, but the windows 'as delivered' are (at least in the case of Chassis #197) permanently fixed in place.
There were a few late Series 1 cars with two-piece side windows without regulators in which the rear portion was held in with spring clips and removable.