It's made in Japan so I'd guess it's a metric,,maybe one of the 2mm sizes (2, 2.2, 2.5mm).
Our #3-56 is one of the most common sizes and very close to those 2+mm sizes.
Most metrics are simply re-tapped to a common US size and a standard replacement installed in US shops.
#3-56 is one of the most common F/Site bead thread sizes, though I've seen them as small as #2-56 and up to #6-48.
#6-48 usually reserved for the larger glow-bead type sites. I think the old Ithaca Ray-Bar is 6-48
There's also the distant possibility that the front site bead is the smooth tapered pin type (unthreaded) that simply force fits into the likewise tapered hole in the rib. Unlikely I think,,but I thought I'd mention it.