This also says that the M939's are just rebuilt M809's???
In David Doyle's book Standard catalog of U S Military vehicles. It says that the M809 and M939 were produced side by side briefly as the M939 series production began and the M809 production ended.
Now I ask why would they be building M809 just rebuild into M939's
I know I'm answering an old post but there are a number of reasons to build both models during the transition such as filling open orders for other nations. Or, needing older models for units who are short trucks but who well down the food chain for upgrading the unit's entire fleet. The last thing you want to is have a few "oddball" trucks in a motor pool with no spare parts or model specific special purpose tools in stock and the addition hassle of training an entire unit's worth of drivers and mechanics for a few units of rolling stock.
And there is good economic sense in consuming the "on-hand" stock of older model parts. Lastly, manufacturers are generally required to keep older model equipment of many kinds in limited production for all of the above purposes. Uncle Sam pays for the production line to sit idle if nothing is getting built. So in a perverse way, buying a few items and keeping the line open and active is actually cheaper in some cases and is always better than losing the institutional knowledge and production facilities needed to rapidly replace losses or ramp up production in wartime.
The most glaring example of all of this is the need to buy an aircraft carrier or nuclear attack or missile submarine when there is no clear and immediate strategic need for one so that the companies and employees who build them are available when needed......
Lance