Some of the later renditions of Coopers centerfires in the model 52 were made to accept a Remington 700 type trigger. On the smaller models (57, 21, etc) there is not enough room on the tang to fit a 700 type trigger.
Some of the early model rimfires, and IIRC, and small caliber centerfires could be had with a Jewell trigger but they are quite rare.
I did look at fitting an Anschutz trigger to a model 57M but there was no feasible way to do it, the tang on the 57M was not long enough.
To the best of my knowledge the only option for trigger replacement on the rimfire, and model 21 is a Jard.
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