[deuce_roadster]

When looking at part number prefixes, the second number if a 1 means the part was originally for a Ford and if the second number is a 9, it was for a Merc. This holds for up to 48 anyway. As Marko39 listed the 81t you can determine that the first use of those heads were on 1938 Fords and the T would indicate truck. 99T would indicate 39 Merc part first used on a truck. I would guess for more power. The letter A in the 3rd position indicates passenger car. I know most people here all know this but it doesn't hurt to restate for people new to the hobby.