Few people remember when noncareer employees (90-day casuals) were part of the bargaining unit and received the same pay as career employees. The four national craft unions, negotiating together, made the fateful decision to exclude the supplemental workforce from the bargaining unit in July 1975. Former Clerk Craft Director Robert Tunstall used to remind people of this.
The Postal Service soon cut the pay of the supplemental workforce and began using them big time in lieu of career employees despite language that they wouldn’t. Thousands of grievances were filed. It took 36 years to return noncareer employees to the bargaining unit, but look at the huge cost.
1975-1978 Collective Bargaining Agreement (new language is in bold print)