The FCU relay is not really a relay, but it looks like one and resides in the XIX socket at the CEB.

Due to a change in instrument cluster and a pickup of the speed reference signal form the rear differential to the rear ABS sensor (and later the front ABS sensor because the found that a spinning wheel made the tach go crazy) a frequency converter was needed. The FCU.

The FCU relais exists in 2 versions,
964.618.220.00
964: 1989-1994 Porsche 911 Carrera 2/4
968: 1992-1995 Porsche 968

964.618.221.00
928: 1987-1991 Porsche 928 S4 (seems wrong to me, should be 1989-1991)
928: 1992-1995 Porsche 928 GTS
964: 1989-1994 Porsche 911 Carrera 2/4

Based on this they seem compatible. This needs verification !!