The coin Auction catalog I have with me I thought had these obscure coins in does not might be in one of my older catalogs in storage?.
I recall prices of Yen 350,000 - Yen 450,000 were the auction start prices for bidding for Genuine coins like this.
Beware chinese coins on ebay as I and other auction members in Japan have seen Chinese fakes of just about every valuable coin on the planet in circulation here as well as chinese fakes of all the famous Brand name antiques & Meteorites fossils etc.
If you paid a couple of hundred for it its probably fake check the seller and See if they have more of the same listed & get Reputable Dealers opinion before you buy.
The Writing on the Verso of this supposed silver slug appears to be a fake Japanese script & This will give Away Fake Japanese coins & antiques made by chinese & these are in World Wide circulation even chinese made Fake gold nuggets, that aint gold so beware.
Fake gold coins that are Heavily plated gold & aged patina.
We See Tons of this stuff in circulation in Japan & lots of it arrives here 1st before its exported to europe America australia etc & sold in flea markets & off the WEB in these countries.
Another aspect to remember about almost all Asian coins is that fakes were produced Hundreds of years ago also, some fakes were produced more than a thousand years ago of coins minted in 750 values rang from $25k upwards, so beware the cheap ones for sale around the WEB ~!" .