I have a structural opening of 915mm. I see you have a frame with a width of 915 mm. Can you confirm the thickness of the frame or door size/width needed for this frame?
The 36-inch external frame is intended for use with a 36-inch external door, but a competent joiner may trim them to suit a smaller door (as cutouts in the head and sill will need recutting, and the jambs may need to be reduced in height). Please refer to the attached PDF file to assist with your query.