Why is version not exported from `ei.h`?

Not really sure what the best category for this topic, but here goes…

I found this weekend about the Erl_Interface (ei.h) and decided it would be cool to have it in my go-to Scheme implementation. I started writing a wrapper library and found it weird that the version constant (131 currently) isn’t declared/defined in the header file. I wonder why?

ei_get_type returns the version as type, but given that it’s not any one of the defined type constants, one has to guess/trust/believe(?) it must be the version?