This begins specifically for the MGA Roadster standard seat frame, being the most common seat in the MGA.

The picture on the right shows a Roadster seat back frame, driver seat for LHD car or passenger seat for RHD car. Notice the small kick low on the outboard side (near the chassis frame rail), and the larger kick on the inboard side (near the tunnel). This allows it to fit between the chassis side frame and tunnel while being wider across the shoulders. Also notice that the top corner is higher on the inboard side so the curved top of the frame nearly follows the contour of the steel body tonneau behind the seats.

Todd Clark has the seat frame base detailed dimensions on page 10 of his book "MGA Owner's Information & Technical Guide". See www.clarkespares.com

This photo compliments of Bob Myers in Sparta, TN, USA.

Notice two different seat bottom styes, likely both factory issue but probably not originally in the same car. My original seats from August 1957 production has the straight sided screens. The taper sided screens are from later production (and most current replacement parts). Seat on the right is missing the hand hole/ventilation slot.

Photos below show a right side seat back frame (taller side toward tunnel) and a left side seat base frame (angled side toward tunnel). For opposite handed parts just reverse the image, as they are mirror image parts.