Basically the cowling. Only aware of this because the missing part for the corresponding mission to test parts, screenshot. I looked up the part on the wiki and it is listed as being under the Advanced Construction. Which I have researched. Also checked the integrity of all files from Steam properties menus. Uninstalled all mods, note no content mods were ever installed. (Only had KER and Alarm, and a calculator which never worked.) Rechecked file integrity still no dice. Not knowing what parts are suppose to be there make you ignorant to your needing them. But I am sure when trying to protect and make things more aerodynamic I would have discovered this missing important part eventually.