So, this is my first-ever pipe, a simple but pretty little "Filter" 65, a gift from an Irish uncle on my 18th birthday:

Since this clearly doesn't hold a 9mm filter, I believe it's from the 1980s Filter line, which apparently featured a "triple groove air filter", according to the book also pictured. Other descriptions fit, but 6mm adapters don't, and 6mm balsa filters get shaved down a bit when inserted. Is my pipe missing a condenser-type filter that was originally installed, or was there a special filter cartridge Peterson sold that was a little under 6mm? Does anyone know more about that line?