how to clean the hour meter?

That is a field replacement hour meter so the 507 hours only represents what's on the new meter. There should be a four bolt window cover over the numbers. Maybe you removed it for the picture??

Chrisco , slacken the large cover on the left of the numbers - remove the complete service meter ( 2 bolts ) from engine ( don't worry the gear that drives the numbers is not timed or marked ) - remove end cap and the number barrel should slide out to let you clean it - the housing should be dry when installing it just put a LITTLE grease on the gears - as magnet says the glass and screws are missing. Cheers Al

OM....how can you tell that this is "replacement" hour meter and not the original hour meter?

The 1957 7U originally had the hour meter with the round window and flip up cover.
The style you have is listed in the parts book as a field installation unit. Service Meter Group 8H5905 which was original installation at 7U41687-up.

Finding a 51 year old tractor with only 507 hours on it is not real likely but it is possible.

Hour meters were known to fail regularly, get knocked off, damaged, etc. and should always be suspect especially if they are replacement models or show low hours or any other reason that the tractor might not match the indicated hours.

The hour meter on my 1952 D2 apears to be full of oil, can't read it anyway. Would this be caused by a seal or gasket issue? Would have to be replaced or can it be repaired. It is only one of the many things I would like to correct now that I have a strong running tractor. Thanks for the help. Tim