I have completed the repair which you mention . I made my panel out of 1 mm sheet steel

I think the flange which you are referring to is the up-turned part , just above the swaged < bit ??
Not having used a commercial part , I assume that the extra steel is to fill in the possible missing steel above the < bit. Suggest you look at the Haynes MM/ 1000 Restoration Manual L Porter pages 101 to 107 , where all will be revealed !
John