The Principal Storekeeper of the Ordnance, sometimes known rather less grandly as the Ordnance Clerk, was an official British civil service position, subordinate to the Master-General of the Ordnance and a member of the Board of Ordnance. This Board was responsible for supplying the Royal Navy and British Army with military supplies, from ammunition to artillery to military maps. The Storekeeper was responsible for maintaining the stores of military supplies.
Although the Board and its officials were abolished in 1855, its legacy continues to this day, in the form of the Ordance Survey, the United Kingdom's national mapping agency.
Thomas Hastings was the historical Storekeeper at the time this novel is set. As well as being a respected Naval officer and expert on guns, he was also a painter.
See here for a list of other holders of this office.