I got this request for help, anyone know more?

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i've a pencil sketch of helms deep & the hornburg which i recently purchased, framed from a car boot sale. it has helms deep sketched in the centre with the hornburg in the background. 'helm's deep & the hornburg are written to the left of centre. it's old, judging by the fading on the mount and it measures approx 14" x 14". i can find 1 or 2 images via the internet but no detail of it's origin. i'm guessing it's from a calendar maybe?

can you help?