The chips marked 512 are 512 Kilobits, they are 8 bits wide so each chip is only 64 Kilobytes, 4 x 64 Kilobytes is 256 Kilobytes. 8x512 Kilobits would give you 512 Kilobytes.
If you take a good look at the manual, you may see an alternative setting for 512 Kilobytes also. If it supports up to 1024 Kilobytes, then it must also take 1024 Kilobit chips. Therefore there would be two ways of having 256 Kilobytes, with 8 256Kbit chips or 4 512Kbit chips, one setting as you see works, the other doesn't. It may be the same for 512Kbyte, which could be 8x 512Kbit or 4x 1024Kbit, you may have previously had it on the 4x 1024Kbit setting.
Shuttle boards of that time were decent quality I think so maybe it will make use of it if you find the right configuration, but remember what settings are working now, in case it really doesn't like those other chips.
