Quoted from G-P-E:KSP42 is an equivalent that was made by Fairchild Semiconductor.
MPSA42s were originally made by Motorola (ON-Semi) but discontinued them a few years back.
ON-Semi bought Fairchild, discontinued the KSP42, started making MPSA42s again (KSP42 with a different part number) ... and now they have discontinued them again.
Fortunately, there are still a couple other manufacturers of these left.
https://www.rocelec.com/part/ONSMPSA42
Back being made