Typos! Must have been 1872 and 1868, these numbers would fit perfectly in the 1700 - 1900 range:
The first WM20 was frame number WM20.111, and the first batch (frame numbers WM20.111 - WM20.251) all went to the Ministry of Supply (contract number unknown). Up until frame number WM20.243 there were still no "matching numbers" (engine numbers were chosen "at random" from the WM20.1700 - WM20.1900 range (*).