A lot of clothes manufactured in the US have either a WPL (Wool Product Label) or an RN number on the label - and these numbers can help, along with our other tools, to narrow down a date of a company.
The WPL numbers were issued by the FTC between 1941 and 1959 - so if you find an item with a WPL it means that the company that made it started between those years. It does NOT mean that your item is that old however, as the company may still be in exsistance and using the same WPL.
The same goes for RNs - these were first issued between 1952 and 1959 - after 1959 RN numbers completely replaced the WPLs.
There's a great entry about RNs and WPLs on The Vintage Pavement - Subhadra explains it really well, and has a great mathematical formula for cracking the code.
Also - check out Ikwewe's guide to RNs, WPLs and CAs - The woman is brilliant.
If your item only has an RN or WPL number and no other labels - you can find out which company owns the number here - and then use TESS to find out more about them.
See how it's all coming together?