Unfortunately there is no conclusive way to determine from a fuzzy photo. The only real way to determine is to compare it side by side with known examples.
The stamp pictured in the original post is perf 11, with the bottom perforations trimmed off.
Find a scanner and scan the possible 518b and a couple of 518 stamps in the same scan. a 518b will appear to be brown
compared to normal 518 examples. Note that two 518b deep brown shades are said to exist.