They are business strike cents from Philadelphia. The ones that are missing the 'S' are proof cents from San Francisco, that are proof coins without the 'S' mintmark. They look like this:
Depending on how many dies were used, there are at least 3K of these, unless they pulled the die early before the full run. So there were 6,851,765,000 of the Normal 1990 Business strike cents that year. These are not in short supply. It is the proof ones with no 'S' mintmark that are the valuable ones.