History & Overview
Arnold was the internal code name of the first Amstrad CPC prototype, which was built around a MOS 6502 CPU. As they choosed the Z80 based programming language Locomotive BASIC as a part of the core system, Amstrad decided to replace the 6502 with the Zilog Z80. One of the "Arnold" prototypes was sold via Ebay in 2005.
The Gate Array Simulator
The Gate Array was the only component of the Amstrad CPC, which was not a standard one but developed by Amstrad. Before it could be produced as a single chip, they had to make tests with a prototype, which was a single print on top of the actual motherboard. Aside from the Gate Array logic, the PCB does also contain the 32K BIOS ROM (stored in four 8K EPROMs).
The Grey Amstrad
The first cases of the CPC 464 had a grey colour instead of the later used black colour. Probably these prototypes have been sent to several software houses to enable them to start developing CPC software before the computers' release.