Steven,
The eyebolts were used on almost every top-level French constructeur bike. Some smaller makers used non-adjustable stays, with their ends flattened and drilled. The disadvantage was that if fender or stay got bent, you couldn't adjust it. I talked to Ernest Csuka (owner of Cycles Alex Singer) about this, and his comment was "C'est idiot!" You need a bit of adjustment - the eyebolts provide a millimeter or two, but that is enough to get everything to line up.
I also asked whether the eyebolts were intended to release the stays in case of something getting caught between fender and tire. He says that in 60 years in the business, he hasn't seen that happen, nor any problem due to stuff getting stuck between fender and tire.
M. Csuka told me that once, riding his randonneur bike in a cyclocross event, so much mud got stuck between fender and tire that the wheel wouldn't turn any longer, but of course, it wasn't catastrophic.
So while I don't know why this is a problem with plastic fenders used currently in the U.S., it is not a problem with alloy fenders, if mounted securely, and at the correct distance from the tire.
And the eyebolts are the proper French way to mount an alloy fender, on cheap bikes and expensive ones alike.
Of course, I have no idea how the Italians mounted their fenders...
--
Jan Heine, Seattle
Editor/Publisher
Vintage Bicycle Quarterly
http://www.mindspring.com/
Furthermore, Jan will concur that many if not most of the top French 'constructeur' bikes come with one piece stay loops and no such eyebolts. I dare say that the purpose of these eyebolts was more than likely solely to allow adjustments to an industrial product that couldn't otherwise be used on another non-standardized industrial product (the bike).
Look at how virtually all the vintage Italian racing bikes from the pre-1960 era came with or had purpose-built mudguards available. Virtually all the mudguards were designed with a specific bike in mind and were very sturdy and had virtually no adjustability. Road debris and rough road surfaces were far greater at the time, so it only holds that any "well-documented" problem would have been far greater then than now. I therefore agree with the comment (I believe it was Jan) that the most important point is to ensure a sturdy mounting and structure. How does the saying go? Light, strong, inexpensive; pick any two you want, because you can't have all three.
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Steven Maasland
Moorestown, NJ