I found this Russian site through another forum, apparently the valve mod which involves enlarging the holes gives quite a power increase, The notches in the mag are to make a ratchet to stop lead BBs hammering up and down in the mag.

http://natsuki.nm.ru/mr654k/mp654k_fix_up_tune.htm

You can put the web page in the url translate tool

http://www.google.co.uk/language_tools?hl=en

but it still mostly unreadable.