Guards Machine Gun Regiment Buttons

[button_magpie]

Hi all,

I've meant to join the BCMBF for some time now as I always end up referring to the forum (in a non member capacity which obviously has its limitations) for all those unanswered questions or when identification of a button or badge has been bugging me for ages. Maybe if I had joined sooner, I would have the answers by now and retained my hair!
If I may, I'll kick off my first thread with a bit of a poser - I've searched the site for any existing threads to no avail, so forgive me for any repetition here.

If a brass GMGR button has no backmark - does this make it a repro? I have an 18mm plain backed tunic pocket version which is in mint condition. It looks very much the part except the brass content appears a little low. I have been informed recently that all GMGR buttons should be marked (e.g. Jennens & Co. Ltd being a typical manufacturer). Furthermore, a batch of plain backs were produced for a TV programme! Could this mean I have acquired one of those?! I'm really intrigued by this and wondered if any other members could shed any light on any of the above.

By the way, I collect all sorts to the York & Lancaster (WW1 period predominently).

Thank you!