Salop is indeed the recognised abbreviation for Shropshire ...
The origin of the name "Shropshire" is the Old English "Scrobbesbyrigscīr" (literally Shrewsburyshire).
Salop is an old abbreviation for Shropshire, and comes from the Anglo-French "Salopesberia". It is normally replaced by the more contemporary "Shrops" although Shropshire residents are still referred to as "Salopians".
When a county council for the county was first established in 1888, it was called Salop County Council. Following the Local Government Act 1972, Salop became the official name of the county, but a campaign led by a local councillor, John Kenyon, succeeded in having both the county and council renamed as Shropshire in 1980