We don't know.
Like you said, there is clearly no visible writing on the lamp in the movie, and it's never mentioned again.
BONUS: Let's go to the source!
In "Aladdin" from 1001 Nights, there is no mention of any writing on the lamp. (The following passages taken from the Windermere Series translation.)
"Descend those steps, my son," said the African magician, "and open that door. It will lead you into a palace, divided into three great halls... At the end of the third hall you will find a door which opens into a garden planted with fine trees loaded with fruit. Walk directly across the garden to a terrace, where you will see a niche before you, and in that niche a lighted lamp. Take the lamp down and put it out. When you have thrown away the wick and poured out the liquor, put it in your waistband and bring it to me. Do not be afraid that the liquor will spoil your clothes, for it is not oil, and the lamp will be dry as soon as it is thrown out."
Aladdin descended the steps, and, opening the door, found the three halls just as the African magician had described. He went through them with all the precaution the fear of death could inspire, crossed the garden without stopping, took down the lamp from the niche, threw out the wick and the liquor, and, as the magician had desired, put it in his waistband.
Rubbing the lamp:
Aladdin's mother took the lamp and said to her son, "Here it is, but it is very dirty. If it were a little cleaner I believe it would bring something more."
She took some fine sand and water to clean it. But she had no sooner begun to rub it, than in an instant a hideous genie of gigantic size appeared before her, and said to her in a voice of thunder, "What wouldst thou have? I am ready to obey thee as thy slave, and the slave of all those who have that lamp in their hands; I, and the other slaves of the lamp."
In fact, there is strong evidence that the lamp didn't have any writing on it, and was in fact plain copper.
The magician wanted to know no more. He resolved at once on his plans. He went to a coppersmith, and asked for a dozen copper lamps; the master of the shop told him he had not so many by him, but if he would have patience till the next day he would have them ready. The magician appointed his time, and desired him to take care that they should be handsome and well polished.
The next day the magician called for the twelve lamps, paid the man his full price, put them into a basket hanging on his arm, and went directly to Aladdin's palace. As he approached, he began crying, "Who will exchange old lamps for new?"
...The magician never doubted but this was the lamp he wanted. There could be no other such in this palace, where every utensil was gold or silver. He snatched it eagerly out of the slave's hand, and thrusting it as far as he could into his breast, offered him his basket, and bade him choose which he liked best.