I don't know how significant it is but Riihimäki did make sheet window glass.
Probably very significant. The first stage in slumped glass making: get your glass
Although they would still need to set up transfer or (probably in this case) silk-screen printing, and then purchase the moulds, this should have been fairly straightforward as all the supplies were probably readily available.
It is possible they could have gone elsewhere, but less likely if they were actually producing the flat glass. I wonder if they ever moved onto making float glass though :?:EDIT
: According to LJ's book, it would appear not.