Beaverstuff, I have two bikes with Ishiwata tubing. Both, ironically, are Ross bikes. I know Ross is better known for their department store bikes, but in the mid 80s they made some nice road and mountain bikes. I do not know the date of the Ross, Mt. ST. Helen MTB in the pictures, but I estimate it to be a 1985. It was a $5 yard sale special, with two new tires and tubes (since replaced) and a Deore LX deraileur (since removed). The decal is in bad shape but, its all I've got in a MTB with Ishiwata tubing. It says the tubing is 4130, triple butted. This bike is not light, weighing in at 34 lbs.
The other Ross is a 290S made with Ishiwata triple butted tubing. Although the frame is too small for me, I rode it for a couple of months this year. It has a very smooth, nice ride. Weight, about 28lbs. Sorry about the quality of the Ross downtube pictures it's the best I could do tonight. I know you're into Fujis, so I included pictures of my 82 Monterey Valite labels. As you can see, Fuji wanted everyone to know this bike was made of Valite tubing (3 labels). Despite the heavy steel rims and steel chrome bars on the Fuji, it weighs the same as the Ross, 28lbs.