Rockwood
In the past, examples of Colonial Queensland arms I have encountered were marked quite clearly with the usual "Q-arrow-G" stamp. I had not observed any with just the "Q - number" marking. Shortly before the recent outage of this site, I contacted a knowledgeable local collector and dealer in things such as that very nice Webley .455" Mk.IV you illustrated and asked his opinion about ownership. He commented that he had seen several arms of different patterns marked with just "Q" and that had been taken as having the same meaning as "Q-arrow-G".
The attribution of the illustrated revolver to a specific unit, however is another matter. The various Imperial Bushmen contingents were raised at the request of the Imperial Government - and Murray tells us that clothing and equipment for all IB contingents were paid for by the Imperial Government as well. Like many Contingents. the 4QIB were issued rifles and bayonets upon arrival in SA. However IB officers were required to supply their own sidearm of Imperial pattern and it had to take Government ammunition. Standard procedure in those days.
I think it would be pretty hard for anyone to definitely say that your .455" Mk.IV Webley was issued to the 4QIB - unless there was anecdotal evidence or it came from the estate of a veteran. As a matter of interest, some time ago I was shown a revolver similar to yours which definitely did see active service during the late stages of the ABW; clearly marked "Aus. Com. H' on it's butt strap. All the owner knew was that a distant relative had owned it and he was very pleased that his unit could be identified.
Best regards
IL.