I’ve noticed that all the modern retro-bikes - the Bonneville range, Royal Enfields and the BSA ‘Gold Star’ - have a rather unsightly broad flange around the front and lower edges of the petrol tank, although the 50s/60s originals had no such flange. Does anyone know why? (On the usual principle of ‘Follow the money’, I assume they are somehow cheaper to make this way.)
Easier to weld and therefore quicker to manufacture.
But earlier T120s also had a seam/flange down the middle:
Actually seen as a great design element these days. Note the chrome edging to accentuate it.
Black rubber edging, or chrome (cheap and in different sizes available on Amazon) is easy to apply and helps with the look if it’s not to taste.
The weld flanges can be removed with the tank re welded, I even think de seamed tanks or tanks without them can be bought but the work/cost won’t be cheap.
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Just like the seams on the original Minis.
BMW didn’t chose to perpetuate that design feature for some reason. 
Those seams would help stop water and spilt petrol running under the tank. Not saying that’s the reason they are done like that but it’s a good side effect. I’ve certainly noticed the water dripping off the seams when washing the bike, and I’ve been know to spill petrol when filling to the brim using fierce pumps.
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