Its possible they want something in the TCP header to help them dispatch the body to the right portion of their system, as they may have a custom TCP stack (why is a completely different question). You may be able to get the required effect by using a IOCTL call. RTFM for what can be changed in a socket via IOCTL, and get them to better define the required portion of the TCP header that needs adjustment.

I know the IP packet has a protocol field to enable 'passing up' of the packet contents to the right layer (generally TCP, but it could be UDP, or a custom protocol), abd X.25 has an 'application specific' field so that registered X.25 apps can listen for packets.