Delta Products built and sold these S-100 systems. Then their name changed to XOR (at same physical location) and may have been because purchased by US Micro. I believe for a period of time bare bone boards were sold as XOR while Delta Product boards were populated. Not real sure anymore. XOR also made early PC clones before disappearing. I have a multi-page US Micro sale booklet and a 1982 price sheet.
More info on the Delta Prod/XOR Z-80 board & 64K dynamic RAM boards - they both used S-100 pin 60 (SIXTN) for MREQ* vs. the more common pin 65. Plus the CPU board can only address 20 address lines and not the full 24 address lines of the S-100 spec's. Easy to change the first one once you are aware of it.