I own this preamp and have all of the printed materials on it. The modules are part of the Boulder modular series that predated the L3ae, L5ae, 2010, 1010 and 1012. There were a number of modules. There is the power supply module (MS01) that can power up to 4 of the other modules. There is the switch module (MS21) which has no active circuitry and simply directs inputs in the line module. There are two phono options (MS10 & MS11). The MS10 is the simplest and allows MM cartridges. The MS11 adds low cut filters and covers MM cartridges. Two MC cartridge upgrades were available for the MS10 & MS11. One is a Jensen MC step up transformer and another is a solid state gain stage that is similar to the front-end of the 2008 phono preamp. There are also two line options. One is the MS32 which has a high impedance (200K) buffered input and featured a Penny & Giles volume potentiometer. The other MS31 is a more standard single stage line section that has a 25K input impedance (this is from memory - I have the MS32). Both had an optional balanced output option. There is a balanced input line stage called the MS51 that allows balanced line level sources. there is also a microphone preamp module also featuring Jensen transformers, but I do not own it and don't remember the MS## model. There are outboard DC power cables that link the supply to each powered module. There are 1, 2 and 4 module wide cabinets to house the variety of modules. I hope that this helps...
