The Labor department’s website has a listing of prohibited-transaction exemptions, organized by EXPRO, class, and individual exemptions.
Under class exemptions, the website shows the history of an exemption, with Federal Register citations for each proposal, adoption, and amendment or clarification. (For example, the display on the Qualified Professional Asset Manager exemption includes eight citations.)
Under “Insurance Agents In-house”, the display shows citations for the proposal and adoption (both in 1979) of PTE 79-60, and nothing further.
https://www.dol.gov/agencies/ebsa/laws-and-regulations/rules-and-regulations/exemptions/class
Here’s the government’s posting of the portion of 44 Fed. Reg. 59018 (Oct. 12, 1979):
https://s3.amazonaws.com/archives.federalregister.gov/issue_slice/1979/10/12/59015-59020.pdf#page=4
If it’s hard to read, better images are available from HeinOnline and other publishers.