Are there two separate firmwares, one for the bulk version and one for the retail, like it was with the 1620, or they're just the same (only one firmware for both the bulk and retail version)?
Yes their is a bulk/retail firmware but some people have gotten the retail firmware on their OEM 1640 drives (which also happened with the 1620).
My 1640 oem drive from Newegg came with the regular firmware (BSGB)
Flashed it to BSLB - and all is good-
I'm sure there is a .cvt file around here somewhere to cross flash to retail if someone ends up with a "G" f/w on a drive
I doubt there is a different retail/bulk firmware for a 1640 at all, it's all the same, isn't it?
Well they have a GSHB firmware on their EU FTP server if someone wants to try and test it but its more than likely the same as with the 1620 B/G differences, BenQ exclusive features removed/crippled etc.
At a hunch they probably use the Gxxx as a basis for the OEM 1640 rebadges.
Like 8T8 said, there is one bulk firmware available, but AFAIK, all OEM 1640 up to date ships with the retail BSxB firmware.