Traced a gearbox oil leak on my mc21 to the shaft seal. Parts manual says it is part 91207-KV3-830. From this blog, this japanese fellow seems to have had the same problem, and replaced it with part 91204-MG3-003.
http://blogs.yahoo.co.jp/kdjjn164/20796086.html

Just want to confirm before ordering that they are one in the same (in my native language...)

fortunately the other one seems much cheaper:
http://www.webike.net/sd/9449704/

cheers