See: L-3 Communications Secure Terminal Equipment (STE - Office)
I believe the "STE Sleeve", which is screwed to the bottom, is used to convert the "STE Office" to the "STE Tactical," adding two red and two black connectors for the TRI-TAC military network, and MILSTAR BDI (RS232) 25-pin connector. This sleeve covers (replaces?) the POTS RJ11 connector. The STE Sleeve has a label on the bottom, indicating Cage Code 1ELF0 (STS International, Inc.), and "STS P/N:10910400". An STE mounted in a small container is shown in STS's history on their website, but is not described.
L3 STE