Many “manufacturers” of musical equipment do not actually make some or all of their products. They outsource manufacturing to a “private label” company that will badge, or brand, the product for them. This occurred within the United States before the trend moved off shore. Harmony is an example of a company that never made an amp, but sold many, many models with their name on them. Gretsch amps were made by Valco at one time, who was a supplier to many other brands such as Kay, Supro, Harmony, and others. Many of these amps share identical circuits and cabinetry, or have only minor changes between brands.
Sound Projects Co. appears to have been a “true” manufacturer. Most of its circuits are unique to Lectrolab amps, and probably designed in house. One, the Lectrolab R400B is identical, except for cosmetics, to the Harmony H306A amplifier which indicates that Lectrolab made this amp for Harmony as well as itself.
Sound Projects Co. also made an amp for St. Louis Music (SLM), a large musical instrument distributor who subsequently created or purchased Alvarez , Crate, Ampeg and other brands. On of the SLM “house brands” was Custom Kraft. While SLM distributed Lectrolab amps in the 1960’s and perhaps the 1950’s, they also had Lectrolab produce a private label amp for them which was badged as a Custom Kraft Electra Bass Amp Model 800. Except for cosmetics it was identical to the Lectrolab R700C. SLM also sold Custom Kraft guitars, probably made in Japan. There are probably other amps with the Custom Kraft label although they have not come to our attention yet.