Matt, the numbers of 1100 and 1300 are the lengths of the springs in millimeters. The Service Manual has the following data (I've copied and edited a little, with mm data in italics):
Front leaf spring
Dimensions (Length x Width x Thickness -Number) mm (in)
1,100 x 70 x 6.5 - 5
(43.31 x 2.76 x 0.256 - 5)
Free camber mm (in)
135 (5.31)
Laden camber mm/kg, (in/lb)
56.0/310, (2.205/684)
Rear leaf spring
Dimensions (Length x Width x Thickness -Number) mm (in)
1,300 x 70 x 7.5 - 5
(51.18 x 2.76 x 0.295 - 5)
Free camber mm (in)
145.0 (5.71)
Laden camber mm/kg, (in/lb)
22.0/535, (0.866/1,180)
This data is for the light duty model that we have in the USA. I didn't include the numbers for the heavy duty to avoid more confusion, and in case needed, numbers are taken from a 1978 Service Manual.