Actually, it was a +6 wand.
A basic +1 magic weapon is a 1st level item. A basic +2 magic weapon is a 6th level item.
I'll admit that I also thought it would go in sets of 4, but if you look at the price chart, it seems pretty clear that the 6/11/16/21/26/31 cutoff is a major price jump, which would lend well to the + of the item going up... since Lurker59 has the adventure and discovered +1 is 1st and +2 is 6th, and the prices for those match the chart. It's pretty good evidence. At any rate, we do have evidence that a 21st level character was using +5 armor from a playtest blog and an 11th level character a +3 weapon, so those are perhaps helpful backup information.