The Swedes originally were planning to use Rolls Royce Medways as they'd used Avons on the Lansen and the Draken.
Unfortunately these were replaced in UK production by the Spey which was too small for the job so they went to P&Wh for the JT8D.
This was reworked extensively into the RM-8 and they developed their own afterburner and thrust reverser (which was a quite a thing in it's own right I believe).
When you think it was designed in 1957- 1963 it is a remarkable aircraft design.