It's funny how Mr. Mack keeps showing up and is still, for the most part generally overlooked when it comes to popular H-D history. Without a doubt he is a significant connection when it comes to the "early" years. It doesn't seem too hard to picture what a Tri-Car would look like in 1906. What a find that would be!
That was an interesting piece on USPS history. It seems like a grey area when it comes to the part regarding Tri-Cars with no specified marque mentioned though. I have seen early ads for the Indian version. It's hard to tell when it comes to the picture link you posted. If I were to guess it looks like a Indian Tri-Car from the teens.