"It's tough one, and I'm pretty sure there isn't an unequivocal answer.

Pen inks have generally been at least partially water soluble, i.e water-based solutions, for hundreds of years. However, historically, pen inks have been colored either by dyes, pigments, or a combination. Dye-based inks are generally more readily soluble in water than pigment/particulate based inks, but it's hard to tell what is what. 19th century pens were generally either quills or metal nibs in nib holders. They generally wrote with whatever ink they had, and it ranged from pretty strong stuff to mild diluted ink. So, the question of solubility in water is probably not answerable. You are correct that truly washable ink was a 20th Century item, produced in the (I think 1920's as a marketing item to advance the use of fountain pens as something one can carry around.)

However, a lot of inks used to create permanent impressions contained what is called iron gall, a true ferrous byproduct. English church registries are still clearly legible 1500 years later are so because of the ink. American founding documents were written, it is believed, with ferrous ink. I have some... so your stamp might have been that. "