79HGK negative voltage regulator from RS Components. Package: 4-pin (gold plated) , TO-3 Download RS Components posthumous datasheet ( really rather good !) : RS. Components part number : 307-878 Status: NOS (New Old Stock) circa : 1979 RoHS: no ***Please read warning contained on RS datasheet*** This is one for the collectors ! you certainly would not want to use it.... is this one in a million, well ... looking at the stock lists it's about one in two million, so quite fair to say it's a rare one. The pin markings and the clipped case are original features.