Mavic 840/841 questions

[James Thomson]

I have a small collection of Mavic 840 and 841 derailleurs I'd like to press
into service:

http://www.bikepro.com/products/rear_derailleurs/mavic_rrder.html

Before I begin experimenting, can anyone suggest the likely capacity (max.
sprocket and max. wrap) for both?

I don't currently own any Mavic indexing levers. Can anyone suggest what
non-Mavic shifters and cassettes might work, and in what combinations?

Finally, there's a little cam with on the derailleur body between the barrel
adjuster and the cable clamp. The cable can pass through a hole in the cam,
or across its curved outer face - the intention was obviously to provide two
indexing options. Can anyone tell me what these two options were?

Thanks.

James Thomson