Originally Posted by microcord

And the drop-outs are of course ready for bolts to attach those full fenders. A bike I'm riding a lot these days has a British frame built in 1961; this too has drop-outs ready for bolts for attaching fenders. However, they're not for any metric bolts. (My uneducated guess would be that they're for BSC.) I don't know when the change to metric threads took place (and presume that it took several years). I imagine that others here will know a lot more than I do.