I've come across this usage, but I can't find it in the usual online dictionaries (Lexico, Collins, AHD, CED, M-W ...). I remember the expression 'young cannon', so I've searched for this.
From an article from 'The Authentic Campaigner', reviewing Three Years in the Confederate Horse Artillery (tidied) by George Neese:
August 25 (1862) The sharpshooters were firing at each other across the river all night; The Yanks made three attempts last night to burn
the bridge but our sharpshooters drove them back every time. This
morning the Yanks on the hill near the bridge were firing swivels at
us. A swivel is a species of young cannon – light, and mounted on a
tripod that looks something like a surveyor's compass. The barrel is
fixed on a swivel or turning point. The ones the Yanks fired at us
this morning threw a shot about the size of a walnut. However I did
not see any of them. I judge the size only by the keen whiz they made
as they sped past us. I wonder what these Pope Yanks will try on us
next – shoot a blacksmith shop or a buzz saw at us I expect. This
forenoon we moved back to our wagons, about three miles from Waterloo
The poster comments:
This is a first for me I've ever read of swivel guns being used by land forces: I've always considered them a ship feature. After all,
what use is a small caliber round against artillery in the field?
[Gary Mitchell, 2nd Va. Cavalry Co. C, Stuart's horse artillery]
So a 'not quite' cannon.
A downtoner usage, not an intensifier. An archaic (the Christie example probably thwarts the 'obsolete' labelling) usage, but still sounding better than say 'He had what could almost be considered an arsenal with him.'