R:160 = Rear 160mm
F:180 = Front 180mm

Is it written on the actual brake caliper or on a brake adapter like in the following photo ?

180 Front/160 Rear adapter

Is the brake 51mm IS (International Standard) or 74mm PM (Post Mount) ?

Brake Mount Standards

IS mounts on forks are 160mm and IS mounts on frames are 140mm
IS brake calipers come in different sizes, front 200mm/rear 180mm, front 180mm/rear 160mm and front 160mm/140mm
The 20mm size difference between front and rear on the brake caliper is because of the 20mm difference between the frame and fork IS mounts.

IS brake calipers (the left caliper is a 160mm front/140mm rear and the right right caliper is a 180mm front/160mm rear)

The 180 Front/160 Rear PM to IS adapter in the first photo has different sizes on it because of the 20mm difference between frame and fork IS mounts.

With PM to IS adapters: 160mm front = 140 rear, 180mm front = 160mm rear, 200mm front = 180mm rear, 220mm front = 200mm rear