These trains cannot be booked online. Reservation is possible at a French ticket office or ticket machine. Just do it when you get to France. Price is €1.50 per seat and they're unlikely to be full on a weekday so long in advance. Booking 1 day in advance will be fine.

I arrive in France on 8 May and will go to ticket office that day. We are from NZ and hope to go to Mont St Michel on 11th for a day trip. Is it possible to book tickets with my email pass if I ring someone? Or do you still think this is enough time to still be able to get a ticket when we arrive. I don't understand ... is 1.50 the fare? Seems way too cheap?

€1.50 is the reservation fee when you have a Eurail (or Interrail) pass.

Does it work like any other NOMAD trains ?

SNCF hasn't got much information about the reservation (it doesn't mention Interrail/Eurail passes at all) : https://www.sncf-connect.com/fr-ch/article/le-train-du-mont-saint-michel-82724

I'm asking because the special train to Pontorson doesn't appear on bahn.de or any other planner (as far as I could see).

(Otherwise it'd be 54€ return and you don't use a pass day)

Maybe this page just isn't up to date? It's 2 years old. DB for example give a departure at 7:32 and a connecting bus at Villedieu-les-Poêles. Return in the evening also with a bus to that station. Total travel time is shorter than on the above web page.

You're perfectly right as always :)

I mistakenly looked at connections to Pontorson-St-Michel but this train doesn't run to Pontorson anymore, the connection indeed now happens in Villedieu-les-Poëles. I can see it too on SBB app.

Then the 1,50€ seat reservation applies.

You'd think SNCF would create another page or something, well...

Thanks for the answer :)

EDIT : I've emailed Mark Smith (seat61) so he can add the info to his website