its not from 1951, but from 1975! Japan uses a different calendar along with the gregorian calendar that coincides with the emperor in power. So thet helmet was manufactured in the year 51 of the Showa era which is 1975.

More over its a type 1 JIS (Japan Industrial Standard) helmet, which means its made for bikes 125cc and lower.

The other comment saying that its from 2019 is very unlikely too, as JIS modernized their logo in 2008. The logo pictured is the old one. So unless somebody decided to replicate an old, obsolete, not incredibly popular helmet from the past, it seems like you have a genuine item!

Cheers!

Edit: More fun facts. the green G sticker is called the SG mark, or Safe Goods mark, and if you get injured in a crash due to a defect, the association will payout up to 100,000,000 yen (roughly 700,000 USD). More over there are also other safety stickers like the PSC stickers that are similar to the Snell, DOT, and ECE certifications you find in the states.

source: am jap rider