The same material as can be seen in as Vac form kit, are also used for detail and conversion kit. These kit that often cost as much or more as the actual kit, are often of a very high quality, and gives one the chance to make something not usually seen. In this scale where the choices are less than in 1:48 or 1:72, it's also nice to be able to build several models from the same basic kit.
These kit also allows one to build models that otherwise only can be build from scratch.
A few comparisons between kit standard parts and a detailset can be seen here
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