At the time I seem to recall this was documented as being the SWITCHAR in config.sys certainly by MSDOS3.x this was well known and I seem to recall trying it ou

At the time I seem to recall this was documented as being the SWITCHAR in config.sys certainly by MSDOS3.x this was well known and I seem to recall trying it out on MSDOS 2.x on a commodore pc clone (in Sweden) this would have been around 1985ish