That looks to me like the numbers for a MEP-701a (MEP-016B with an ASK), it is my best guess that the T89 and T90 refer to 1989 or 1990 either construction date or revision date for the MEP-701a housing. the -24P manual for the MEP-016B is TM 5-6115-615-24P which matches the partial number that can be seen in the photos. My MEP-701a has a T-89 number on it, when I removed the top cover to gain access to the unit, the MEP-016B cover below shows a 1988 built date.
Ike
P.S. for some reason the MEP-002a and MEP-003a just added -A or -ASK and were field retrofit for the ASK sound suppressing housings, where as the MEP-016B was renamed MEP-701a when the ASK was added as a depot service operation. This may be due to the ASK installation being more complex, requiring installation of electrical cooling fans, threaded inserts in the frame, etc.