The B+W Push-On Lens Cap #300 model is their 'standard' unit, and has a slightly convex front surface to prevent the inside of the cap from touching the glass of your lens when it is tucked away in a bag. The sides are their 'standard' depth, to fit on normal lens barrels or standard-style filters, and this particular unit has a 54mm inner diameter.
B+W also makes a 'shallow' sides model, the #305 model, to fit on slim filters, and a more common snap-on cap.
It isn't an exact science due to all the variables involved, but generally a push-on cap that is 2-3mm larger than the lens' filter size fits just right.
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Comments about B+W #300 54mm Push-On Lens Cap:
It fits perfectly the rear element of my used Nikkor SW 65/4.0 which came without its original
lens caps.
Pros
Cons
Best Uses
Comments about B+W #300 54mm Push-On Lens Cap:
I use this on Leica X1 instead of the pricey cap supplied by Leica.
Comments about B+W #300 54mm Push-On Lens Cap:
This B + W (Schneider) cap, (SN 223-25 inside the cap; 54 MM, 300, 69701 on the box) fits over the Leica X1 knurled ring. I have no idea if it is safe to use on X1 (instead of the supplied pinch-type lens cap) or if any other 54mm push-on cap will fit.
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