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You must be referring to the ink stamps in your passport that you received at the border, which have a space for "class" and "until." Immigration officers often leave those spaces blank. Those stamps are not visas but admission stamps. You should not list these on your visa application.
A US visa is a full-page sticker placed in the passport before you travel to the US. These always show the type and expiration date. (Several decades ago they were stamped into the passport with ink, and those visas could be indefinitely valid, but they were full-page stamps and they also always showed the visa type.)