It consisted of small self contained bakelite boxes housing everything from VCOs & filters to multiples & envelope generators. These were all battery powered (9V DC).
Aimed at the school market they were sold with a teacher's manual.
They are regularly described as a "poor man's" ARP 2600/Odyssey because many of these circuits were found in those synths.
These modules are extremely rare today.
My collection includes:
The MSL keyboard
Speaker/Amp (Model 1511)
Multiple Box (Model 1510)
ADSR Envelope Generator (Model 1501)
VCA (module 1509)
VCO (modules 1504) x3
The mixer/atenuator (Module 1508)
sample-and-hold (module 1507)
Ring Modulator (Module 1505)
I'm missing the filter.:-(
The sliders are pretty stiff. Needs cleaning and lubrication as with much of my old ARP gear.
The Bakelite case:
I'm on the hunt for the MSL teacher's manual and the filter. If you come across either please contact me.