another indication that it is an Auto 5 is the cartridge/magazine cut-off lever that appears to be in the photo. I didn't know they were on that early of an Auto 5.
I have an early FN Auto 5 that has only a 4 digit serial number, 35xx. The frame has a 1903 patent # stamp so there weren't many built between 1900 and 1903. It doesn't have the mag cut-off. An interesting note from the barrel roll stamp, Barrel (or whole gun, it doesn't specify) was made with something called "Cockerill Steel".