Either it is stored in the ships cargo as bot (check that) or you indeed scrapped or lost the shell.
If so go to the Outer fields again and you'll get a lower grade but sufficient replacement there. If you don't, then the game has identified a shell in your possession (check cargo, warehouse, parts once more)
In worst case, open console and enter:
GAME.lab.parts.append(GAME.sbPartsData["SBPShell2TypeI"].clone())