Possibly "bulleted blank"
Please forgive me for pointing out that if you know about these then your age may be showing. As indeed is mine for knowing that you're right about them.
That said I don't think there was a muzzle attachment; ISTR that the use of bulleted blanks required the use of a special barrel that had a cruciform "cutter" built in at the muzzle, but I could be mistaken. Certainly later weapons (L1A1 SLR, L4A4 LMG (a Bren derivative) L85A1 & 2 and so on) used blank firing attachments to enable the use of non - bulleted blanks.