The "EM1" area with Y1-Y3 and U2 usually has a metal plate mounted on the visible spring clips.  The heat sink that is normally mounted on the processor plate stands just over 2 inches high.

  The 591 uses the S5 memory card, system can handle 2GB of memory.
The Model 591 is a quad-word system if two memory cards are installed and an eight-word system if four memory cards are installed. Total memory cards in the machine can only be two, four, or eight.  Memory must be installed in combinations of two, four, or eight cards. 

  The first two (if only two cards installed) or first four (if four cards installed) memory cards installed must be installed in pairs and of equal capacity (for example all 32MB).  The second set
of four memory cards installed can be of any available supported capacity but also must be of equal capacity to each other and installed simultaneously. 

7013-591 XIO Planar FRU 73H2604, P/N 73H2605.

U2 81F8890
U6 Toshiba 81F9018
U19 PAL,  73H2606
U22 65G7530
U24 00G2743
U26 Toshiba  81F8125
U29 Intel N80C186XL20
U30 81F8929
U33 51G8834
U34 LSI  81F9021
U91 51G9846
U1F (on back)     40H2913
 

J28-35  32-bit slots (J29 VBE)
J36 4x82 connector to CPU planar
J37 HD 2x30 pin connector for keyboard/mouse/tablet/serial 1/serial 2/parallel panel
???  Diskette drive connector (can't find a label)
J38 50-pin SCSI connector
J39  HD 2x30 pin connector for keylock/3-digit display/reset button/battery panel
J40,41   Power connector         

Y1  14.3181 MHz oscillator
Y2  24.0 MHz oscillator
Y3, Y4, Y9   ??? crystals labelled 100 PD9F
Y5  32.768 Khz crystal
Y6  9.216 MHz oscillator
Y7  39.95 Mhz oscillator
Y8  31.95 Mhz oscillator