I making Tamiya's SU-122 for B&B campaign and I have a problem at piont 4. Should I use part D9 and external fuel tank or only D9 or only tank? My intuition (and parts configuration) tells me I should use either part D9 or tank.

You can either fit the drums or the cradles, you don't need to fit the drums ON the cradles. Don't mix the drums and cradles like on the box art as this page SU-122 , shows all drums or all cradles, but not both together.

Note that one of the SU-122s in the last picture has some all-steel wheels AFAIK this is the only picture that shows this combination.

Hope this is helpful

Drader is right. The tanks already come on their cradle, so the use of both would be incorrect. However,photos are always a great reference, but the lack of a photo does not necessarily mean the lack of something existing. Many T-34 based vehicles can be seen with a missing fuel tank or two. There are any number of reasons why one would be missing. They are held on with quick release devices, and fairly easy to remove.

Thank you guys. You're very helpful.