First I will correct two misleading staements in my original post.
(1) "There is no special information other than a check-in-the-box that a tax was due or paid." By this statement I meant that the record of poll taxes in Georgia tax digests did not convey any more information than the number of polls paid - this could be for oneself as an adult white male under 60, and could also represent the number of slaves (under age 60) that the taxpayer owned. Tax digest do contain a wealth of other information about taxpayers besides poll taxes.

(2) Starting in 1868 all adult freedmen tend to be recorded by name in tax digests, but this does not mean (as I implied) that they were not recorded by name in 1866-1867. Indeed, self-employed and land-owning freedmen are commonly named, and others are occasionally named as employees -- it is just that the tax digests did not consistently name freedmen untiol 1868.

Piano and pocket watch taxes!! How imaginative! But Georgia had quite an antebellum history of taxing luxury items, including stallions, four-wheel carriages, roackaways and coaches, "sulkies & buggies." The special county taxes you mention may have been levied under authority of the following law:
ACTS OF THE GENERAL ASSEMBLY OF THE STATE OF GEORGIA, PASSED IN MILLEDGEVILLE, AT AN ANNUAL SESSION IN NOVEMBER AND DECEMBER, 1861.
PART I.---PUBLIC LAWS.
Law Number: (No. 74.)

Full Title: An Act to authorize the Justices of the Inferior Courts of the several counties of this State, from time to time, in their discretion, to raise such extra taxes as they may deem necessary to equip Volunteer or other soldiers from their respective counties, and to provide for the indigent families of soldiers; to authorize the collection of the same, and to legalize their previous action in the premises, and for other purposes.

By the way, I copied the text of this law from a website maintained by the University of Georgia and accessible through the Georgia State Archives, at the link below. (HINT: unless you are a resident of Georgia, this is the only way to get to this website without paying a subscription fee).