If You look closely, then all SAC-build Flankers have their construction number noted on the intake and like Blitzo noted correctly, the first two numbers are denoted to the batch - I often call them Block similar to the ones used on the F-16, however batch is probably better indeed ! - while the second one shows the aircraft's individual number within that batch.
Deino
PS: by the way, Huitong has now '111' too !
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Yeah, I think it's better to call them "batches".
Even for F-16s, "block" typically refers to capability upgrades rather than production run per se (such as Block 50/52 referring to F-16C/Ds)