I am finding it tough to get replacement for the transistor K2717 .31. There are assorted K2717 transistors on the market but with different sub codes other than .31 as indicated in the picture below. Does the sub code matter?. Any suggestions please?

 

Per the Toshiba data sheet I found here:
https://www.promelec.ru/pdf/2SK2717.pdf
That is simply a lot number.

 

As stated already, that second line is either a lot number or a date code, depending on maker. If the description matches the description of the 2SK2717, and the pin functions match, (g,d, &S) then another device with the same number should work. Just be careful bending the leads, and remember the heat transfer material.

 

Another vote for not worrying about the second line. Matching the first line is lucky itself.