1987 XJSC windshield compatibility

Does anyone know if a 1990 XJS windshield would fit a 1987 XJSC?

You should look at VIN number of your banger first. Year 90 for XJS is a year of miracles, also know as experimental transition to facelift:

This one goes to XJS coupe, apparently up to 1991, with some exceptions. You can easily notice the rubber seal and locking strip all around

This type apparently is later model - glued in and apparenlty slifhtly thicker, after 1991

Knowing British automotive industry - I’m expecting both being same shape/size apart from thickness…

Your car would have a rubber seal. As long as the 90 has a rubber seal it will fit. The glue in windscreen is a bit larger and has the black adhesive receiving material around the edge.