This story is adapted from a story in Adventures into the Unknown (American Comics Group, 1948 series) #26 (December 1951) with the same basic script and new art. Other printings of this story include: Horror Tales (Eerie Publications, 1969 series) v9#3 (August 1978).
While 'El agente de la National, detective y gourmet' had its own series in 1986, this seems most likely to be from Mortadelo especial (issues 210 and 211 at least) due the the credited involvement of Zentner.
"©—Editorial Vilmar—Barcelona—The Illustrated."
A "TrueVision" story.
Other printings of this story include: Scary Tales (Charlton, 1975 series) #44 (May 1984).
Other printings of this story include: Weird (Eerie Publications, 1966 series) v2#3 (June 1967); Terror Tales (Eerie Publications, 1969 series) v1#10 (November 1969); Tales of Voodoo (Eerie Publications, 1968 series) v3#6 (November 1970); Strange Galaxy (Eerie Publications, 1971 series) v1#9 (April 1971); Terror Tales (Eerie Publications, 1969 series) v4#7 (December 1972)
"©—Editorial Vilmar—Barcelona—The Illustrated." Other printings of this story include: Pánico (Vilmar, 1978 series) #22 (June 1980) [as El monstruo misterioso].