There is no, and has never been, an interior kit for a straight M109. The Trakz kit was also for an M109A2.
Your best bet is to get the Black Dog M109A2 interior and use it to backdate to an M109. Most of the interior parts were the same. The BD set is built for AFV Club's M109A2 though, and will not fit Italeri's M109 well. I have the BD set and an AFV Club M109A2 that I plan on backdating using Italeri M109 parts. I think this will get me the most accurate and best detailed M109 model.
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