Aha! So after a night of sleep and that link (which I had seen, but had misapplied) it dawned on me that the examples given were are for your_domain/urbackup whereas I am hosting from the root of just your_domain.
With that knowledge secured this one minor change addresses the issue: remove the /urbackup/ part of the path from the location parameter. 
location /x {
include /etc/nginx/fastcgi_params;
fastcgi_pass 127.0.0.1:55413;
}
}
I’ll run for that for a while and report back!
Thanks!
S