Traditional IRAs don't have a "cost" basis, they have basis in nondeductible contributions. Since nondeductible contributions were not permitted until 1987, a contribution made for 1985 did not add to basis in nondeductible traditional IRA contributions, even if you failed to deduct a permissible contribution.
Having a Form 5498 for a particular year just means that you made an IRA contribution for that year. IRA contributions first became permissible in 1975. I'm not sure when Form 5498 was introduced, but it was before 1987; I have one from 1986.