My understanding was that the three unmanned launches prior to Apollo were designated Apollo 201, 203 and 202, because that was the number of their space vehicle. Grissom's flight was officially called Apollo 204 in this vain. But that just wasn't sexy enough, so they unofficially adopted Apollo 1 as their name. After the accident, for some reason, NASA decided to sex up the naming switching to sequential numbering, which would have retro-name the already flown flights as Apollo 1, 2 and 3, with Apollo 4 being the unmanned test which under the old system would have been called Apollo 501. But the widows petitioned NASA to honour their late husbands' desire for their ill-fated mission to be known as Apollo 1 and NASA acceded. They officially posthumously renamed the mission Apollo 1 and due to the awkwardness with retronaming now, they just left it as is and only continued forward with Apollo 501 as Apollo 4.
