Some of your M lenses have small white double-digit numbers next to the infinity marking on the lens barrel - what do these mean?

These numbers tell you the exact focal length of your lens and they only appear only on the M 50, 90 and 135mm lenses. What you do is drop the last digit of the nominal focal length and then append the two white numbers to give you the exact value. This is best explained by the following examples:

My 50mm Summicron has the number "22" engraved on it - this indicates the exact focal length as being 52.2mm. Another 50mm lens has 98 written on it, making the exact focal length 49.8mm. A third lens (a 90mm) has 00, this means that the lens has a focal length of exactly 90.0mm. Pretty easy.

This, along with the older, more cryptic, #2-#8 precision coding scheme, was discussed in more detail on the greenspun.com Leica forum at <Greenspun.com: #005cTt>

So why bother? What does it matter? Dennis Painter provides the following explanation:

[…] The precision of the Leica M system rangefinder is such that two lenses which have an actual focal length variation of even a tenth of a millimetre require a different helical mount to translate the focus extension correctly to the rangefinder assembly within the camera body. Leica lenses have always had the focus helicoid matched to the lens head. In the past the 50mm helicoids had code numbers stamped under the infinity lock. Each number representing the exact focal length for which the helicoid was cut.

[This is the cryptic #2-#8 scheme I note above.]

[…] There are two reasons for variation from the nominal focal length. In some cases the actual focal length used for the lens computation may be different than the nominal. The best example of this is the original Summicron which had a computed focal length of 51.9 mm. (note, I no longer have a reference but this is from memory). The second reason for focal length variation is due to manufacturing tolerances. Grinding lens elements to the exact curvatures and thickness specified in the lens formula is not economically feasible. Thus slight variations within manufacturing tolerances can result in the lens having a focal length slightly different than the formula.