" The top cover had no serial number and the bottom cover backside has no manufacturer location markings - no Germany or Singapore, nothing at all. The lens ser. no. is 2306916, a pretty low mfg. number. What's up with that? Is it pre-1976" The "Made in Singapore"-print on the camera back got rubbed off.
"I just bought a used Rollei 35 made in SG and there is a serial no. 6256277 printed next to the viewfinder. Is this something common?" Yes and no. Rollei 35 Made in Songapore with a serial number in that range are common, number 6256277 should be "unique"