Because I just know that people were concened about this, I thought I’d just issue an update. I have in fact replaced the front lens-cap for my EF 17-40mm f/4L USM.
gotcha!
I sadly lost the front lens cap at Bristol cricket grounds while having fun getting sunburnt, being english, drinking pints, and watching cricket. It feel down the stadium thing, and while I could see it, it wasn’t accessible 
I dunno why I waited so long to replace it, but I will say that the lack of a lens cap hasn’t really affected anything at all. I got a replacement 77mm front cap, sadly not ‘Canon Ultrasonic’ marked on the Isle of Wight. Bit of a long way to go I guess, but it was only £2.99. And at least it doesn’t say Nikon.
