Nice catch.....but on earlier photo, it looked like a “7”
http://graphics8.nytimes.com/images/blogs/thelede/posts/suspect-number-2.JPG
here’s a link to the hi-resolution photo. Looks like a “3”.
http://thelede.blogs.nytimes.com/2013/04/18/new-higher-resolution-image-of-boston-marathon-suspect-emerges/