Yesterday while commuting from Tambaram to Adyar/Mylapore through M15 and when the bus was near Jayanthi/Thiruvanmiyur, I just observed a H21 in front of me through windshield. Right at the same moment and exactly in the same line, there was a 21H volvo coming in the opposite direction. For a fraction of the second since the two buses were aligned exactly in the same line, the bus nameboards could be read as follows

 

H21 21H (a kind of palindrome :))