My point was that the armament for these ships was NOT built to order. That is, the intent may have been to arm with one type of gun, but the guns and related equipment (e.g. ammo boxes) were often scrounged from many sources. Last minute changes were made based on availability. Remember, at this time, the merchant vessels were competing with the new Fleet ships for arms, so the merchant ships generally took a back seat and had to use what ever could be dug up in time to launch them.
So, even though this ship may have sailed with 20mm guns, it is definitely possible that she may have been originally intended to have 50's, or they may not have had 20mm ammo boxes available and instead used 50's. This sort of horse trading happened all the time, and the merchant ships usually ended up on the short end of the stick, so to speak.