I just had a look in Nigel Browns book British Gunmakers vol 1 London
Serial number 8999 is 1887
they state that numbers 9,000------10,000 unused and 10,000-----10849 missing
rook rifles 10,850--10,999
" " " 13,000--13,999
" " " 17,400--17,999
the above block of numbers have no dates

Upon looking in Colin Greenwood's book .The Classic British Rook & Rabbit Rifle
a review is done on rifle 9454 which is one number before yours
In the same chapter it states "The problem of identifying the real maker of any rook rifle are exacerbated by the fact that H & H records of rook rifles are incomplete"

Even though a rifle may have been made elsewhere the supplier had to meet Hollands high standard.
" The rifles were made to H & H specs and were rigorously checked ,shot and regulated at the factory before being accepted."
I hope this may be of some help
good shooting
sbs470