To my knowledge, the Universal carrier was introduced after Dunkirk, (a) to make up for losses, (b) as a cost saving/production method of combining the Scout Carrier and "Bren Carrier", which were identical in every aspect except the rear body. The Bren had sloped panels on one side, and was open on the side behind the driver. The Scout was reversed, but the rear panel was horizontal.
I enclose a couple of shots of conversions I made to the Tamiya 1/35th kit.
In my opinion, a picture paints a thousand words.
BTW I am still trying to find out how the rear panel was shaped, as very few photos exist of the scout carrier showing this detail. I have been through the resources at Bovington, and turned up very little on the scout.
One other major difference, the Scout was fitted for radio and attached to armoured formations, the bren was attached to infantry formations.
The Scout had wire mesh over the exposed rear of the engine to suppress radio interference from the engine.
Hope this helps.