A quick search in the web, resulted that J.P. & S.W. was Union Works at Willenhall.
No J.P. alone. Anyway another member (Paul the grenade) may be of a help here.
Nos 5 & 23 were obsolete by the end of WWI and No 36 was used.
I am seeing this as "JR". Cannot help, sorry. Not listed on the Mills Bomb site.
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All the manufacturers are not in the Mills Bomb site, I have other manufacturers who are not on the website of David Sampson.
Sorry i can't help further...If Paul, who is an advanced collector of handgrenades, sees your thread may help out...
Take care, Thanos.