I have a 550 Euro Varmint in .308. Same action as the 557 but in a laminate Monte Carlo style stock. If you’re planning on rebarreling, I guess you’d only be concerned with the action/stock?
The 550 Euro Varmint or 550 Varmint Laminate uses the same push feed action as the 557. Not sure why they didn’t call it a 557, but they didn’t. You are correct that most all other 550 models are of the CRF Mauser variety.
also the CZ mags are 2.87” internal length, since I didn’t mention before.