I have a 175TS same as Ian, it too has this type of sprocket and I'm having a hell of a job locating one too!
Nigel Lacey told me that they were indeed fitted to the early models with Grimeca hubs, but that he too has been unable to find a source. As Ian has already said, they are fitted to other small 50s and 60s Italian marques too.
I'm going to have a word with Talan Engineering in Yeovil, UK this Autumn when I take the bike off the road for a final fettle before the Giro next year. They supply Nigel Lacey with his 'normal' sprockets and although they weren't overly enthusiastic when he approached them previously about making a small batch of these unusual ones. I'm hoping that if another person also asks the same question, they may change their mind. Nigel has already said he would take some if that helped make the numbers work.
I'll let you all know how I get on in the next couple of minutes months. At the moment with the weather good, I'm having way too much fun riding the bike to take the sprocket off to show anyone
