I have a “roughly” 1965 US One Sheet catalog (Pentel of America - 1965 One-Sheet.pdf). It doesn’t have a specific date, but it is prior to the S55 and MEC, but the Graph is on there.
I also have a 1967 Japanese One Sheet (Pentel of Japan - 1967 One-Sheet.pdf). It has the Sharp 7, S55 and Graph. I estimate it be 1967, since the MEC (which came out in 1968) is not on there. Plus on the Pentel Timeline presentation, the “S5” shows to be released in Showa 41, which should be 1966.
So, roughly 1966-67 is when the S55 came out. The last catalog I see it in is the US 2011 catalog (Pentel of America - 2011 Consumer Catalog.pdf).
You can grab any of the above catalogs from my website Pentel Catalog PDFs - #3 by nimrodd