Very interesting.
Here is a excerpt from the 1878 Atlas of Halifax that shows a 51 Argyle Street, but no 50. It looks like there is a hotel and carriage factory where 50 would be.
This address is on the block where a new convention centre is being planned. The Entire block is currently vacant, so I think it's safe to say that the 50 Argyle no longer exists.
Here is a link to the entire 1878 atlas if you are interested.
http://www.archive.org/details/cihm_13042