ready reserve was a creation by our previous iam chairman, back in the 90s.
Although the iam continues to bargain ready reserve and weekend part time and separate pay scales or health benefits for part time, it does so at major airlines as well in various fashion but stays away from the term ready reserve now. At United it has temp workers which resemble ready reserve except their duration is capped to around 100 days plus extensions.
The iam signed a contract outside of bankruptcy with Parker in 2008 that had ready reserve so I wouldnt be surprised if there was some form of temp agents in this contract.