From proxy server :
2016/10/17 14:19:38 [error] 6735#6735: *236 open() "/var/lib/nginx/html/000001" failed (2: No such file or directory), client:ip, server: localhost, request: "GET /000001 HTTP/1.1"

From srv1 & srv2:
- [17/Oct/2016:14:16:47 +0000] "GET /123456 HTTP/1.1" 301 185 "-" "Mozilla/5.0 (X11; Linux x86_64; rv:49.0) Gecko/20100101 Firefox/49.0"

nginx (proxy) conf:

location /123456/ {
rewrite /123456/(.*)$ /$1 break;
proxy_pass http://SRV1/;
proxy_redirect off;
proxy_set_header Host $host;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_max_temp_file_size 0;