Is my component LTC6904 correct?

Hello, I am Srijan from Biotech Vision Care Private Limited, a medical products company based in Ahmedabad (India). 

Few years back, we used a part of Linear Technology which was probably This Link

Please check the attached image to understand this better.
This image indicates the backside of Older IC and the New IC

The front side of the IC has written "LTAES e3". The backside of the IC (used earlier) has written - "Malay 6C55" and "Malay 7G60". We are using these two ICs to generate Frequency of 40 kHz (Ultrasound) and 2 MHz (Diathermy) respectively. These ICs have been working perfectly fine. 

We recently made new boards of same kind and ordered the same IC, the top marking on IC is same, but on backside this is written - "Malay 8B97" and our board is not working. We are sure that IC is different because when we replace this IC with IC from an older board, the board starts working.

I need help understanding what do these codes on the backside indicate? Are they just batch numbers or do they correspond to a specific part number? And why the latter IC is not giving desired output, is it an altogether different part? Which is the correct part that matches the exact codes?