Presumably this is an American thing, when a guild or union is referred to in the news or TV show/movie it’s often something like “Local 200” or “Local 600” etc.
For instance in The Blues Brothers 
What’s the number referring to? Members? ID number?
It’s not something I’ve encountered in Australia or seen in media from other countries.
The big unions in the UK have (or had anyway) branches identified by numbers. When I became a transport manager, I had to deal with the militant 5/35 branch of the Transport and General Workers Union, under the infamous Alan Law. When I was appointed, all the drivers were actually on strike.
It’s the chapter number of the particular union. I was in the SEIU which had chapters all over the country. Each one had its own number to distinguish itself.
Yes it’s the chapter of that union. For example every city might have a plumbers or carpenters union. New York is number (x) Chicago is number (y) Boston is number (z) all belonging to the same parent.
Unions are also big on seniority. At least in the unions I’ve been a member of, the chapter numbers are assigned sequentially as new chapters are added. So the low numbered chapters are sorta like Charter Members here on the 'Dope.
Not that the low number gives a senior chapter any greater formal power on the national level than any other more junior chapter. But it does confer bragging rights.
Thanks, makes sense now.
Well, you should have demanded they show you their completed 27b/6 forms. Where we would be if we didn’t follow the correct procedures?
And even within a city, there might be multiple chapters. In a city with multiple steel mills, for example, each company could have its own local.