Very likely that particular boot code was proprietary, in case of need there is the Etherboot/gPXE project:

http://etherboot.org/wiki/

but there is a whole world of different Eprom sizes and the usual possibility of incompatibilities with the NIC or with the BIOS on those old machines:

https://tldp.org/HOWTO/Network-boot-HOWTO/a628.html