if it helps, the serial number on mine is H61241. It sort of looks like there should be one more digit, but the Warranty states it's for model number 61241.
I believe the ones made by National with the "National" name are older than the ones made by National with the "Eldredge "name, however the ones made by Eldredge with the "Eldredge" name are the oldest of all.
I read on another site someone with a 4 digit serial number that there was made in 1913, but the person stating that didn't have any reference information to back it up. I know this model was made for many years, so you may have one of the earlier ones, and I have one of the later ones.
My manual has a date of 10-'25. When I first saw the add for mine, the seller said it was made in 1930 and I figured they were wrong based on everything online I read about them - but you can't argue with a date stamped Warranty Certificate LOL!!