(moderators - move this thread or delete it if you feel it's unsuitable)
Part 1:
http://www.youtube.com/watch?v=fSINDMAF-NY
Part 2:
http://www.youtube.com/watch?v=yqw81QP_w98&NR
Now what got my interest was the price of the 50 sen - some 15,000 yen which is around 120 USD!

That is my example that I have, except that it's Showa 12, not 13.
While the comedian mentioned that it was minted in silver that made it a high price, I obviously knew that wasn't the case. I did a brief check on the catalogue.
Interestingly enough, mintage numbers are relatively high - for Showa 12, about 48 million were minted and for Showa 13, some 3.6 million were minted. Interestingly enough, even with a high mintage of 3.6 million, it IS a key date! Makes you wonder why doesn't it?
Remember in 1939, World War II broke out. That was the very last year that silver coins were minted. To make matters worse after the loss of war, Japan was rumored to have melted a fair amount of silver and gold coins to pay off war debt although no one has any idea how much was melted down. There are also indications that the allies consficated a fair amount of precious metal coins as a reserve.
In 1957, silver 100 yens debut again only to end in 1966. There is also no indication that I know of, if old silver coins were melted for this, in particular war silver coins as it was deemed that anything related to the war should be destroyed.
Regardless, kudos to the grandfather - he does seem to know the importance of that coin! Geez, I mean, he is an expert in so many things, I wasn't really expecting him to be "partially" a numistist as well!
Anyways, maybe good news for the metal detectorists?