MK-131 is the number of the interface. It's not a model number. The MK-131 interface was used on many Acoustat speakers. The Spectra series have a slightly different interface, the MK-2123. There are also two versions of the MK-131 interface. The later versions (Medallion) are slightly different. There was a factory upgrade to the Medallion version.
The actual speaker model numbers are all derived from how many esl panels are in each speaker. For instance, Model 2's and Spectra 22's have two esl panels per side. They are generally ~22" wide. Model 1's have one panel and are ~12" wide. Model 3's are roughly 28" wide with three panels. The very tall (7'10") models are stacked versions of the shorter (5'6") models Their numbers are, 1+1, 2+2, 3+3 and even 4+4 although few of the 4+4's were sold.
The best I have ever been able to find is a schematic for the MK-2123 interface. I have a bunch of other literature from Acoustat. Let us know what model you actually have and I may have some factory info for them. A picture would be even better.