The Duetto was built in 1966 -number ...1158. While digging through the Duetto box of parts, I found the sprayers. To my surprise, there was a wee collection of seat belt mounting bolts. I picked a pair with no shoulder, withdrew one bolt and the 'new' bolt screwed right into the transmission tunnel. I did not attempt any others but the threads seem to match up with the pair that fit.
Thanks Mark but I feel certain the the threads will be different so I wouldn't ask you to go digging. What year is your Duetto?
Can you take a detailed photo of the sprayers?