First, go to the "Illustrations" section and find the brand and model of the car, specifically. Find the items in the illustrations you are seeking, then note the number (__-__-__ format) at the end of the arrow line to the part. There might be a descriptinve word there, too.
Second, go to the part numbers section of the catalog and find that "group" number and description section. Across the top of the page, you'll find the platform designations for the various brands and model numbers of the vehicles (top of the column). You can find this information, usually, in the VIN decode in the front "Infoermation" section of the parts book. The part of the VIN decode that you'll be interested in is the brand/model and body style. As PH23 being "Plymouth High, 2-dr Fastop in 1968). From there, once you have the left-hand parts name and group number section found, find the model designation in the top menu, then see where the two "lines" intersect to find the part numbrer, hopefully.
Hope this might help,
CBODY67