I notice that most "aftermarket" oil filters have fewer "holes" by the mating-face; four holes rather than six. Is this an attempt to restrict oil-flow and thereby maintain oil-pressure? I run a 2011 1388cc Duratec with just 50,100 miles. Should I be looking for a "six-hole" filter? 

I've never noticed any difference in number of holes, a more restrictive filter would cause low oil pressure post filter and high pre filter, it certainly wouldn't be an advantage. The oil works best when it flows freely. Having a few oil filters at home for a few family vehicles I have noticed they can appear identical thread size and face but state they don't fit different vehicles. I wonder if that's the degree of filtration or just so they can charge more when for example a Merc filter would fit a Fiat and vice versa. 

I'd assume it's simply cheaper to make less holes...aftermarket parts aren't usually trying to improve on OE parts.