Mke-2 mic caps
 

I've got two caps in my Sennheiser Mke-2 gold kit, which according to Canford's site "slightly modify frequency response, for matching to individual voices."

http://www.canford.co.uk/ProductResources/ig/4751.pdf

Any idea which does what? (There's a short, and a long... and a windshield of course)

The shorter of the two caps would be for "flat" (normal) response.
The 'presence peak' cap would give you more high-end. ( treble ) For example, when hiding the mic under a shirt, (which tends to muffle the sound) is the most common use.
The 'flat' response cap is 'recommended' for normal outside the shirt in plain sight mounting.
That said, I use the +4dB crisp response cap on my Countryman lavs 90% of the time. Hidden or not

Thanks for the info Rick.

Sorry, this is not correct.

The flattest response is without any cap at all.

The small cap adds a small presence peak.

The long cap adds a larger presence peak.

It's all in the owners manual with diagrams.

You can get a pdf of the manual HERE.

Scroll down to near the end for the diagrams which show the frequency responses with caps and windshields.