there are a few dates for modern Bentleys, such as 1980 Bentley #146,300 but the problem is that dating pianos purely on the basis of numbers is not as easy as it sounds, partly because most pianos have several numbers inside. I doubt if 828501 is the serial number. Can you post photos here to show what the whole piano looks like, or email them to me to post?
It may also be useful to see how and where the number is written. If you want to search for clues inside the piano, have a look at