Several branded sets of speakers were available to match the Prologue 21, though are extremely rare. Production numbers for speaker sets are likely in the hundreds.
SKS-7000 is a small, plastic housing single driver speaker set likely intended to be mounted high up behind the singer.
SKS-7100 is a visually distinct speaker from most competitors, featuring two drivers, the larger being 20cm.
SKS-7200 is visually identical to the SKS-7100, but with 25cm speaker drivers.
SKS-7300 is equivalent to the SKS-7100, but is a much more conventional looking speaker. The Victor SP-P77 is very similar appearing and more readily available, though is not an exact match.
SEGA Prologue 21 