I was in Okinawa recently and came a cross a sign in Hiragana that had a combination of characters that I hadn’t seen before. I’ve been able to read and write Hiragana for many moons and have only seen it this one time.
When my Japanese friend read it, I thought they were mistaken. It was on a sign and I thought it was とう (tou). The う was small but as it was a sign I just figured it was an artistic rendering. I was like "isn’t that “tou”. They were like "no that’s a chichai う so it’s pronounced とぅ (two). Mind blown. First time for everything.
I don’t recall seeing this in any of the Hiragana charts. Have you?
Here’s the sign for the restaurant for the curious.