Answer
The jnx-chas-defines (Chassis Definitions for Router Model) MIB has a jnxProductName branch for every Junos OS device. The system object ID of a device is identical to the object ID of the jnxProductName for the platform. For example, for an M7i Multiservice Edge Router, the jnxProductNameM7i is .1.3.6.1.4.1.2636.1.1.1.2.10 in the jnxProductName branch, which is identical to the SYSOID of the M7i (.1.3.6.1.4.1.2636.1.1.1.2.10).
For a list of supported standard and enterprise-specific MIBs, click SNMP MIBs and Traps Reference