I recently bought this from a friend in Singapore. This is a 4th series RM50 solid number 1. The 4th series issued in 1981 was the last issue with the old design with tiger head water mark. It was issued for a year before the new design known as the Blindman Series was issued in 1982.
This issue was very limited as in those days, the serial numbers were only 6 digits, not like today, 7 digits. So it takes 10 prefixes to form a prefix a nowadays.
Furthermore, many of the high denomination notes(RM1000, 100 and 50) were never released into the market, or collected back in 1982 and were destroyed later on.
The best solid numbers I have seen so far is the Fresh UNC solid 8 under my friend's collections.