Formal description of KDF1 and KDF2
FCD 18033-2
Encryption algorithms — Part 2:
Asymmetric ciphers - the final committee draft (FCD) of ISO/IEC 18033 - by Victor Shoup is publicly available from his website and specifies (in section 6.2) that KDF1 is

For an octet string $x$ and a non-negative integer $l$, $\operatorname{KDF1(x,l)}$ is defined to be the first $l$ octets of
$$\operatorname{Hash.eval}(x || \operatorname{I2OSP}(0, 4)) || ··· || \operatorname{Hash.eval}(x || \operatorname{I2OSP}(k − 1, 4)),$$
where $$k = \lceil l/\text{Hash.len} \rceil $$

The binary counter representation should translate to 4 bytes in big endian.
KDF2 is identical but starts counting from 1 instead of 0.