George you may be right, because if the initials are upside down to the other stamp, and it is a partly worn I, then this could very possibly be an early Sixten Ivarsson.
I can
guarantee it's not an Ivarsson pipe. Early Sixten Ivarsson pipes from end of the 1940’s and beginning of the 1950’s were classic English shapes like Zulu and Dublin, but more slender and slim, with slightly different curves.
Some examples, and good reading (see the comments on markings) here,
https://pipedia.org/wiki/Ivarsson,_Sixten
This same type of shaping can be seen in some of the early pipes Sixten Ivarson designed for Stanwell, see catalog here,
https://pipedia.org/wiki/File:Stanwell1.pdf
I can say with absolute certainty, that your pipe is from Preben Holm's workshop.
"IIS - Also known: IS, IM. Seconds from the Preben Holm workshop."
The S is an abbreviation for the Danish term for seconds, which I do not recall, and I don't speak Danish.