From the Thorpe UK Catalogue (page 14, current one):

"A wild colony in Europe builds comb for brood at 35-38mm centers. This is deep enough for worker brood cells, with enough space between facing combs for nurse bees to tend larvae oon both combs at the same time. For frames that the queen will lay in, whether deep or shallow, this spacing must be adhered to. In the wild, colonies build combs for honey storage much wider than for brrod and often with drone sized cells. Less wax is used and more honey is stored for far less effort. The beekeeper may want to keep all frames though the hive at the narrow spacing, or use wider spacing for honey supers by using wide ends."

So, two questions: 35-38mm for the brood? Thought it needed to be 35mm exactly?

Other thing is, how thick does a feral colony build its honey stores, and would it make sense to have wider bars for that part, and if so, how wide should they be?