This is the first version of the MOS, released in December 1981 with the initial launch of the BBC Micro. The earliest machines were shipped with the MOS on four EPROMS, soon to be replaced by a single chip. Not all features of the MOS are present yet, which means there's still over 700 bytes free in the 16K ROM. Some of these unused bytes give a glimpse behind the scenes, including this text:

 : OPT MOS70
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 :   (c) 1981 ACORN Computers Limited
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 :   BBC Microcomputer Machine Operating System (MOS)
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 :   Change record:
 :   032 12-Aug-81 PB     Swop '_' (no

This is the start of some source code, starting with a change record. PB is Paul Bond who was in charge of the group of programmers writing the MOS. The only other snippet of source code is :SETSDF LDA, where LDA is a common opcode and SETSDF is the label in the original source that I call .claimSerialSystemForSequentialAccess .