help with makers embossed mark...

[villagenut]

Was lookin through some bits and pieces from a site collection box and was stumped as to whose mark this was . It is from what was prob a black glass ale bottle, on the base it has what appears to be....IS-P..... sorry thats all I got, but sometimes it is enough for those who are more expertly in tune with these puzzles, not like myself apparently on this one..

 

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