The dies that struck your coin likely had some excess grease on them in those areas. When the coin was struck, the grease was trapped in those receeses in the dies and couldn't squeeze out, so the metal couldn't squeeze in.
Mint mark on the obverse says it is clad. 68,78,88 or 98. MM was diff in 68-78, and coin quality was poor in 88. I am saying this is a 1998-D. (25 cents) Actually the mint mark alone says it's a 1998. They were smaller and more filled in D's on those other years. If you look with a loupe can prob see the shadow of the 9 in that spot.
Missing digit could be from a grease filled, or debris filled die. The more letters and numbers that are missing the better. I would put this in a 2x2 flip and keep it, but it doesn't carry much if any value over .25 cents.
After being able to take a better look with the enlarged picture I am going with it being from 1988. The mint mark style had more of a primitive look in 1978 and while the mint mark looks more or less the same for 1988 and 1998, the hairstyle became much more spaghettish by 1998. So, based on the style of the mint mark and its less spaghettish hairstyle, I am confident with a high degree of certainty that it dates to 1988.
Yes could be 88 or 98. If I could look through a loupe at this coin with a bright light, I would be able to see the missing numeral.