The RFS-12P-2 have a 1/2″ NPT tapered thread which is almost indistinguishable from 1/2″ BSP.
The switch can be fitted from outside the tank by screwing it into a 10mm (max) deep, treaded boss.

Specification

Contact Rating: 50VA
Max. Switching Voltage:                300Vac
Max. Switching Current:                0.5A AC/DC
Dielectric Strength: 600 V DC
Contact Resistance: 200 mOhms
Insulation Resistance: 10 mOhms
Temperature Range: -40˚C to +120˚C
Max. Pressure: 5 bar
Float SG:   0.70 ± 0.04
Material (all s/steel): Float = 316; Body = 304 grade
Thread (Internal type): 1/8” BSP
Thread (External type): 1/2” BSPT