For The Love Of Port

I have a bottle of Dow's year 20 old Tawny gifted to me. I'm thinking of offering it at a charity auction but need to know it's value AND age. The bottle has no age upon it has a single front label rectangular with an the top of the rectangle arched. The bottle has a neck label with the numbers 861451. The is damage to the neck label at the bottom which may have had a date on it. On the back label is another number, L2189.
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Can anyone help me determine age and is it still likely to be drinkable?

There usually is a "Bottled XXXX" date somewhere on the front or back label. If not, you'll need to post a picture of the bottle so we can try and estimate it's age. (If you have issues posting a picture you can email it to me by clicking the email link under my username).

The value at retail right now, for a current bottle, is about $45 (USD). Not a lot in the world of fine wine.

(Moderator edit: I've moved this to the Port Forum where more people will see it and I deleted the redundant post)