I noticed today that the OD&D LBB's use the phrase "support and upkeep" in two paragraphs. The first instance is the Men-At-Arms paragraph of the Construction of Castles and Strongholds section:
"Hired fighters can be men, dwarves or elves. Chaotic characters may wish to employ Orcs; Orc support and upkeep is only half that of a man. Men-at-Arms require support and upkeep as follows..." (U&WA 23)
The second mention of "support and upkeeps" is the Player/Character Support and Upkeep paragraph:
"Player/Characters must pay Gold Pieces equal to 1% of their experience points for support and upkeep, until such time as they build a stronghold. If the stronghold is in a wilderness area all support and upkeep costs then cease, but if it is in a village or town not controlled by the player/character then support and upkeep payments must continue." (U&WA 24)
Support and upkeeps, then, appears to be a particular type of tax paid to the local ruler and earmarked for the men-at-arms who defend the local castle and surrounding countryside.
There's been some question through the years about how often characters should pay this 1% upkeep cost. Based on the fact that castle owners pay their men-at-arms on a monthly basis, I suggest the players characters should also pay their 1% upkeep monthly.