Looking on google images it seems there was two different variations. My guess is that you have parts from two different kits.
Imperial Guard Demolisher Cannon Mantlet (210-21-01)
A metal Imperial Guard Demolisher Cannon Mantlet, supplied in one piece. This is a Second hand item, please see second hand terms in terms and conditions for full details.
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Imperial Guard Demolisher Cannon Barrel (305-24-08)
A metal Imperial Guard Demolisher Cannon Barrel, supplied in one piece. This is a Second hand item, please see second hand terms in terms and conditions for full details.
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Imperial Guard Demolisher Cannon Top (210-21-02)
A metal Imperial Guard Demolisher Cannon Top, supplied in one piece. This is a Second hand item, please see second hand terms in terms and conditions for full details.
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Imperial Guard Demolisher Cannon Bottom (305-24-09)
A metal Imperial Guard Demolisher Cannon Bottom, supplied in one piece. This is a Second hand item, please see second hand terms in terms and conditions for full details.
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Imperial Guard Demolisher Cannon End (210-21-03)
A metal Imperial Guard Demolisher Cannon End, supplied in one piece. This is a Second hand item, please see second hand terms in terms and conditions for full details.
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These guys stock them if you really want the other end.
http://www.bobnbootzmodels.co.uk/acatalog/copy_of_Second_Hand_Metal_Bits_and_Kits.html#a4736