what does 1939 mean in SK Hynix HMA851S6JJR6N-VK NO AC 1939

I want to increase my RAM from 4gb to 8gb by adding 4gb ram of the part number given in the subject however i am not getting the exact specification. My current ram has HMA851S6JJR6N-VK NO AC 1939 part number and what i am getting online is HMA851S6JJR6N-VK NO AC 1931. So I don't know if i can use that part and will not cause any problem. Hence want to understand what does "1939" in the part specification stand for.

Thanks in advance for the the help.