U22-U25 Toshiba TC511665Z-80H 40 pin ZIP VRAM Functionality
J7 has a little sticker over it that says "J18"
8557SX Bus Adapter
If you have a fantasy about swapping a Lacuna riser and planar into your 57SX, forget it. The Lacuna riser has two mounting slots where the screws are both horizontal, while the 57 has a vertical screw and a horizontal screw. The lacuna mounting holes at the front don't match.
Dotted area on J1- solder pads for a BVE segment
J10- Solder pads for a 16 pin header
J2 AVE slot
Yeah - most have them ... and they are somewhat connected to the video-chip glue logic on the planar. But basically they are non-functional. If you look on the board you will find the same old boring Inmos RamDAC as on the Mod. 50/60 55/65 70/80, with limited bandwidth. Just enough for standard-VGA or 320x200 / 256 colors CGA mode. And - as said: the machine lacks the video modes in its BIOS AFAIK.
Some of the later Greenock-variants of the 8556/8557 planar even lack these sockets and have only soldering spots. That was what my comment refered on.
You might however try to install some chips there for 512K and see if you can enable the mode 2Eh - that's 640 x 480 / 256 colors from a 256K palette at 60Hz vertical / 31.5 KHz horizontal. The system should choke out at Mode 12h ( 640 x 480 / 16 out of 256K, 60Hz / 31.5 KHz) and 13h respectively (320 x 200 / 256 out of 256K, 70Hz / 31.5KHz).
Very friendly greetings from Peter in Germany