Has your question ever been answered?
I do not know Scott numbers, but the difference in numbering makes me suspect those are the "normal" 15 March 1955 (chestnut stamp) and the 1960 (reddish chestnut) Barcelona philatelic exhibition stamp of 31 March 1960.
Due to the long period during which the stamps were in use, variation in the colour exists.
The clue for which stamp it is, is at the bottom of the stamp. The normal has the printer's imprint F.N.M.T. (Fabrica National de Moneda y Timbre). The Barcelona exhibition one has F.N.M.T.-B (B for Barcelona) at the bottom.
The 1956 cancellation on your left stamp makes it the 1955 "normal." The cancellation on the right stamp does not help and the cancellation obscures the imprint. However, the "F" of "F.N.M.T." aligns with "OR" and not "CO," also making it a "normal" from 1955.
Edit: added 1960 stamp colour as per FESOFI website.