Thats only an extra 5 seats, not even one row.
What is meant by ABX and are the 50C seats supposed to be extra legroom ?
ABX is the same as the A300 (or AB6) and just refers to the aircraft when it when it is divided into premium and economy cabins.
When a Premium cabin is made on Monarch's A300, a seat is taken from each row at the front of the aircraft to make it a 2-4-2 configuration instead of its usual 3-3-3 configuration. There are 7 rows that make up the Premium cabin, however row 1 already has a configuration of 2-3-2 and row 7 is made up of just 3 seats , so these rows are not affected. Therefore just 5 seats are removed from the front of the aircraft to make way for Premium.
The front of all Monarch's A300s (i.e. the first 7 rows) has been extra legroom seating since the beginning of S'11.