Before MPK, Bala played back mostly for Kamal (EDKL, Saagar). I guess his voice was identified with southern actors?! MPK was the real break for SPB into the HFM.
My theory: SPB was chosen to sing play back for SK because his Hindi pronunciation had a slight accent (like Saroja Devi's Tamil) which was perceived as "boyish" by North listeners. This worked for SK's character in MPK and HAHK. That's why I asked the story behind "SPB for MPK"