Can somebody explain this?

That’s what it says.....

Many moons ago several companies offered a kit version of their amps. Dynaco, Heathkit, Luxman, etc !

So Macintosh did as well?

abery some good info here and yes for one year apparently 1960:

http://www.audioaficionado.org/showthread.php?t=30811

Keep in mind two would be needed for stereo , and very limited numbers were made .
Dynaco stereo 70 being the best selling kit power amp of that era .