It looks like the label inside the case has "...12' as the scope number, can you make out the remains of the rifle number, which seems to have been scraped off?
As you can see you're lacking the "T" stamp on the receiver wall and the "T Less Telescope" part has obviously been scraped off the chest, so it is unlikely to be a service fitting. Thousands of these rifles were sold as surplus in the 50s and 60s and dealers did all kinds of funny things; like shipping scopes in one container and rifles in another and never bothering to match them up when they got to this side; not that that really applies to this one of course.
One surplus dealer in the UKbought some thousands of MkI scopes from the MoD in the 50s and used to sell them for £5.00 each in the case with the bracket right up to the early 1980s, though the price had gone up to about £35 by then IRRC. So there was lots of this stuff around for those that wanted to put a set together.
If it was, this must have been put together early on for there to have been an un-numbered bracket with the original finish on which to stamp the rifle number.
I would have guessed that this bracket was too late to have been fitted to one of the trials rifles, but the fact that there was no number on this bracket, until someone later stamped the present one on, might suggest the scope and bracket were on one of the early No4(T)s as those did not have the rifle number on the bracket at all.
Anyway, congrats on having the earliest known MkI scope; a remarkable survival in that condition. If Peter hadn't told us it was destined for a Bren gun I would have assumed it started life on one of the 1403 No4(T)s that were converted at Enfield in 1940-41 from the trials No4 rifles. We've discussed a number of those rifles on this forum.