Lantiq xway mips CPU 34kc or 24kc?

I have a Buffalo WBMR-HP-G300H with an old version of openwrt. I'd like migrate to lede, so I see the differences between old and new config.
I see in CONFIG_DEFAULT_TARGET_OPTIMIZATION the parameter -mtune=24kc and in CONFIG_CPU_TYPE 24kc.
In my previous configuration I have 34kc and also in openwrt's wiki there is 34kc.
Why there are these changes? Also in other profiles with Lantiq xway the cpu is 24kc and not 34kc.
I don't know the differences between 24kc and 34kc, I'm a newbie on mips architecture.

Thank you for the clarification.

The 34 core should be backward compatible with 24 object code. The ar71xx directory and packages are all compiled for MIPS24kc, even for models that have 74kc chips.