Looks like it is a little over 1-1/4" diameter. Remember that machined screw threads are rarely the full specification diameter, the crowns being rounded or slightly under max diam. for clearance.
That would make it more likely be 33mm X 3.5mm .
Is it a relatively new lathe or one that is likely to have had areplacement spindle made.
Looked like a 33x3.5 to me as well. I see you have a chuck beside it, assuming chuck fits is there not a stamp on it somewhere with the thread or insert size?
I checked the chuck and the faceplate and neither have a stamping for what thread is being used. There is also no markings on the lathe spindle or around it either.
Looking at the specs here, they mention M33x3.5mm so I am just going with that. I also measured it with a digital caliper and it seems close enough.
For some reason, I had in my mind that each model could have a range of spindle thread sizes (you choose when you order), so it didn't occur to me to look up the spec.