Hello suedo. Welcome to the forums! It is measured at 26' bumper to ball. Minus the A-frame that would give about the 23' interior length you report.
R.J. Dial's Vintage Airstream Home
provides this link to '63 Overlanders - 63Overlander
. There are multiple models to view.
His sample VINs in the upper left corner matches yours exactly -- at least the first Ohio example does:
26 = length
3 = year
T = twin bed layout
316 = sequential production number
Are there any ghost-y red numbers on the front end cap? This would indicate past WBCCI involvement. We have a few members here who have the old WBCCI yearbooks and can make a guess on who may have owned it back when.
Pictures are worth a thousand words. We enjoy seeing these road queens!