I originally though they were (rather bad) fingering suggestions, but then I saw the second bar where the fingering suggestion is 2 4, yet this (51) still shows. What exactly does it mean then when the numbers are in brackets like this? My music theory book does not explain fingering notation at all for some reason, and Wikipedia is not helping here either.
In piano this parenthesis with 2 fingering means that you should change finger while keeping the note played.
For example at the second bar you show, you should first use your 5th finger to attack the note, and switch to your 1st finger while keeping the note played, so you can reach the next note (that is quite far) with your 5th finger.