Cistophor-tetradrachm of Pergam kingdom

Orient555

Greetings to all!

There is a question about the tetradrachm-cystophor of the Kingdom of Pergamon, minted in Ephesus.

Here is the link to the auction:
https://leunumismatik.com/en/lot/42/1123/[42]

The description indicates the year of minting - the 20th, that is, 140-139 BC. But I don’t see a letter indicating the year on the coin.
Tell me, how did the year become known?

Sapyx

The letter "K", located in the right-hand-side picture at 11 o'clock, is the Greek numeral for "20".