Well, the back will have two 'sights', it's just that the dots will be different. On the 'three dot sights' there are 2 white dots, as paulrabe mentioned, on the back sight. The Pro versions use the 'straight 8 sights' where there is one white dot on the back sight, in the middle below the slot.
My PT140 has a 'Z' for the second character, I bought it in '06 and it's a Pro. Fair to assume yours is a Pro also. There is a post here explaining the options of the serial number, but I couldn't find it. If memory serves me correctly, the first digit is the caliber, the second is the year of production and the third should be the model type (the last digit i'm not sure of...).