later models by the colour of the keys – light grey for Issue 1, blue-grey for later machines. Although the official service manual states that approximately 26,000 of these original boards were manufactured, subsequent serial number analysis shows that only 16,000 were produced, almost all of which fell in the serial number range 001-000001 to 001-016000. An online tool now exists to allow users to ascertain the likely issue number of their ZX Spectrum by inputting the serial number. The Sinclair models featured audio line in and out, in the form of an "ear" and "mic" socket. An external tape recorder was needed to load the majority of software released, or the ZX Microdrive. Either socket could be connected to headphones or an amplifier as an audio output, although this would not disable the internal speaker. The original ZX Spectrum model experienced numerous changes to its motherboard design; mainly to improve manufacturing efficiencies, but also to correct bugs from previous boards. Another issue was with the Spectrum's power supply. In March 1983, Sinclair issued an "URGENT" recall warning for all owners of models bought after 1 January 1983. Plugs with a plain (rather than textured) surface were at risk of causing shock, and were asked to be sent back to Sinclair's office in Broad Lane, Cottenham. It's not known how many power supplies were returned, and how many still exist in the wild. Sales of the Spectrum reached 200,000 in its first nine months rising to 300,000 for the whole of the first year. By August 1983 total sales in the UK and Europe had exceeded 500,000 with the millionth Spectrum manufactured on December 9th. ZX Spectrum+ Planning of the ZX Spectrum+ started in June 1984, and was released on October 15. This 48 KB Spectrum (development code-name TB) introduced a new QL-style case with an injection-moulded keyboard and a reset button that was basically a switch that shorted across the CPU reset capacitor. Electronically, it was identical to the previous 48 KB model. It was possible to change the system boards between the original case and the Spectrum+ case. It retailed for . A DIY conversion-kit for older machines was available. Early on, the machine outsold the rubber-key model 2:1; however, some retailers reported a failure rate of up to 30%, compared with a more usual 5–6% for the older model. In early 1985, the original Spectrum was officially