Gibraltar Envelope Stamp

This one penny stamp looks like it might be an envelope stamp. Is it valuable?

Not an envelope, postal stationary.

Valuable? in a collecting sense, yes.
in monetry value no.
Mutilated Stationery (Newspaper Wrapper)

Why would some of these stamps have centimos overprints? Was it connected with a Spanish quarrel over ownership of Gibraltar (which is still going on today)?

Hi Frank.
From 1 July 1856 until 1 January 1876 all mail to Spain required postage to be prepaid by Spanish stamps and there issues were supplied by the Gibraltar Postal Authority acting as a Spanish Postal Agent. The mail forwarded under this system was cancelled at San Roque with a horizontal barred oval, later replaced with by a cartwheel postmark showing numeral 63. From 1857 combination envelopes showing the 2d ship mail fee paid in British stamps and the inland postage by Spanish issues exist.

http://www.gibraltar-stamps.com/history.aspx

Thanks, Rod. All very interesting.