Um... probably OEM replacement head/housing. Not that I'm an expert on the topic, or anything, but it makes perfect sense. Somebody busted the housing back in the day, and they got an OEM replacement part, which wouldn't have a serial.
--a Hoover dealer may have cleaned and polished the then used Mo 541 for resale and removed the original scratched and worn label.They could get new labels and could have,but didn't,stamp the old ser no on the new label.
This is you second time asking the same question and all the answers are within the Thread you started September 19, 2018. Your Post No. 393546. You can find the original serial number by reading all of the Posts on that Thread.