Heffay, the shape of those pipe look exactly like the ones on my bike.
I was also wondering who made them. Mine are slip fit over the header pipes.
My friend has another 350F with pipes shaped the same, but the baffles are totally different inside so who knows.
About the "Yosh" 350F, um, no. I think there would be something aside from some magazine article stating what it is. Hopefully there will be no new guy here saying how "I bought this excellent rare bike off of ebay for only 5 grand, what do you guys think of it??? It's made by Yoshimura!!!".