In the book "Four Years On The Western front" by Aubrey Smith aka "A Rifleman", he mentions buying a replacement set of battalion buttons from the Regiment's Tailor's when he was on leave. He was with the London Rifle Brigade. Many of the London Regiment battalions had their own buttons available by private purchase. The London Irish Rifles did - I have seen pictures of LIR groups with some wearing purchased buttons and some GS.
I've been working on this too as the buttons are one of the ways you can differentiate London Regiment men from KRRC and RB men when the cap badge is unclear.
So far I have ...
7th Londons, (Shiney Seventh) "7" on grenade, "City of London" in scroll, wreathed
9th Londons, (Queen Victoria's Rifles) St.George & dragon below crown
13th Londons, (Kensington Rifles) Name around shield, crown
14th Londons, (London Scottish) Name around crown on star & St.Andrew's cross
15th Londons, (Civil Service Rifles) Standard rifles strung bugle under crown
16th Londons, (Queen's Westminster Rifles) Shield, portcullis, chains & crown
18th Londons, (London Irish Rifles) Name in scroll below shamrocks, harp & crown
19th Londons, (St. Pancras) Name around crown & "XIX"
20th Londons, (Queen's Own) "Invicta" in scroll below rearing horse
28th Londons, (Artists' Rifles) "Artists" then "Artists Rifles" in scroll below classical heads
I'm still trying to determine which of the above were always blackened, sometimes blackened, or never blackened!
I also have pictures of some of the above wearing what look like the standard rifles strung hunting horn blackened button, so the regimental variant was not always worn.
The King's Royal Rifle Corps used the standard blackened strung bugle with crown button.
The Rifle Brigade variant has the bugle inside a wreath, and sometimes included the regiment name, but I have also seen the standard KRRC button described as "Rifle Brigade" - but that might just be ignorance or confusion!
I also have some Scottish Rifles photos with blackened buttons which do not quite look like the standard strung bugle rifle button. I think it's a smaller strung bugle below a crown with a thistle? wreath. They're a little too indistinct to be certain though.
As far as I have ascertained, the Royal Irish Rifles wore the same style button as the London Irish ... with the name changed obviously!
Lastly I'm assuming that some of the other TF rifles battalions - Leeds Rifles, Robin Hood Rifles, Isle of Wight Rifles etc. - probably also used rifles buttons, but as yet I've got nothing conclusive on that.
All-in-all the exact usage of rifles buttons in WW1 seems to have been rather inconsistent, confused and unpredictable! Every time I think I have something definitive, I seem to find a photo that contradicts it! Wainfleet's point about it being "unwise to be categoric" is very well made!
As you can see, this is all very much "work in progress" still, I'm afraid!