Late engines where laser etched in a dot matrix style. They fade / rust easily. They can some times be seen with a camera

New crate engines never had a number. The dealer was meant to stamp the number in on exchange but time is money and the punter is non the wiser anyway.

Never seen an Rs crate motor.

Factory recon, as above.

Only other reason would be dubious providence.

If it has the Rs water pump, larger Rs modine oil cooler, Rs oil seperator heat shield it’s likley to be an Rs engine. Yes you can bolt these to a Zetec but they don’t come up for sale often. Only other way would be to tare the engine down and check the important bits.