ENID BLYTON letter authentication assistance needed.

I recently aquired a Pepys card game dated from around 1952 (Smugglers & Treasure) featuring the Famous Five by Enid Blyton. In the pack are all the cards and the rules, but... there is also what appears to be an original signed letter from Enid Blyton. It has a typed address at the top right hand corner (14-17 St.Cross Street, London, E.C.1.) and the rest is handwritten in black fountain pen. I have compared the signature to other examples actually by Enid and it is exact. The paper is watermarked Titan Bond. My question here is, can anyone enlighten me as to if this letter could be original or if it was mass produced and put in with all these card games?