Charity store staff say the suitcase was donated anonymously and filled with books.
But hidden in the lining of the luggage was the impressive 6ft (1.8m) drawings of the former Royal Navy vessel.
While unusual finds are not uncommon for the charity shop, Ms Parker said she quickly recognised this was something out of the ordinary.
"When I found it I just thought, wow, this is amazing," she said.
"I can only imagine perhaps the family of someone who had passed away donated the suitcase, that is what often happens. perhaps they did not know the plan was there.
"We are all very excited here about it and we've certainly never seen anything like it before."
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