I agree the plan is there for the #1978 GT kit, however that plan shows NO part numbers nor no dimensions of the actual tube length.
Was the 12.5" confirmed ? That seems awfully short to me.
That sounds like the correct length of the MX-Missile tube, which was in the same 'era' and used the same fin unit as the Titan II and Gemini Titan kits but was far shorter.

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When in doubt, WHACK the GAS and DITCH the brake !!!

Yes, there is such a thing as NORMAL, if you have to ask what is "NORMAL" , you probably aren't !
Failure may not be an OPTION, but it is ALWAYS a POSSIBILITY.
ALL systems are GO for MAYHEM, CHAOS, and HAVOC !