“an 1000”
‘1,000-year-old’ is an adjective describing the stone trophy.
I would go by the NOUN which is the subject of the sentence, in this case.
I was trying to recall a case where the number would be the noun/subject, and what I would use then . . .
“a 1000...” works as well. It sounds better. That is how I say it. Feel free to gird your loins and join me on the field of battle. ;-D
Oops. That post to Gingis was meant for you.