I am assuming one of the numbers refers to low windows and other to high?.
Hi Paul,
SR coaches to the uninitiated can take some working out. The diagram number refers to the coach layout rather than high or low windows, once you know it becomes quite easy

.
Diagrams 53, 165 and 317 are 1924/25 'Thanet' stock all built to restriction 1 (8'6'' wide). Diag 165 is the five compartment brake third with low windows.
SR coaching stock diagrams 2001 to 2656 were built between 1927 and 1934. The brake third coaches of interest are below:
Diag 2101 is a restriction 4 (9'0'' wide) four compartment brake third with low or high windows depending on the order number and when built.
Diag 2102 is a restriction 4 (9'0'' wide) six compartment brake third with low or high windows depending on the order number and when built.
Diag 2103 is a restriction 1 (8'6'' wide) four compartment brake third with low windows.
Diag 2104 is a restriction 1 (8'6'' wide) six compartment brake third with low or high windows depending on the order number and when built.
Diag 2105 is a restriction 0 (8'0'' wide) six compartment brake third with high windows.
As you may be aware SR coaching stock carried restrictions denoting the operating areas and routes based on the coach origin (LBSCR, SECR, LSWR and SR) and ability to fit within the respective route's loading gauge. In general:
Restriction 0 - 8'0'' wide - go anywhere - South Western, Central and South Eastern divisions including the Hastings line.
Restriction 1 - 8'6'' wide - go anywhere - South Western, Central and South Eastern divisions - except Hastings line.
Restriction 4 - 9'0'' wide - go anywhere - South Western, Central and South Eastern divisions - except Hastings line and other specified routes in the South Eastern division.
The Bluebell website provides a full description of all of the restrictions -
Bluebell Railway - Carriage Technical Pages - Restrictions