Phrobis Int'l

[kracken]

I have a bayonet and the marking are:

M9
Phrobis Int'l
Spain

on the left side and

U.S.A.
PAT. NO
4,821,356

on the right side. I looked throught the posts and was unable to find any discussion on these. I read on here that they were made for commercial use only. Are these of the same quality as the others? What is the value. I bought it new in 1995 and have used it gently since then. TIA.