I would have thought it was "byseddfwrdd". Where does "bysell" come from?
bysedd - fingers
bysell - key (as on keyboard, typewriter, musical instruments such as saxophone, etc - not as in lock)
Dolch yn fawr. I couldn't find "bysell" in any of my dictionaries. I must look in an on line dictionary.
gweiadur.com is a good one.
Diolch yn fawr