Engel only lists the sheets for four teams (Braves, Flyers, Hawks and Giants) in the guide and states that only one copy of each is known to exist. So my Giants sheet is either that one known copy, or it is a newly discovered second copy. I'm leaning towards the second as being more likely, given that I bought it from an antique dealer located in Kobe who probably dug it up locally. Either way, these are pretty hard cards to find.
The backs of the cards have pictures of dogs wearing baseball caps in red. Hence Engel gave this set the nickname "Dog Wearing Baseball Cap". Which is pretty cool.
