Hi,
Not 100% sure about the current situation but to my understanding there would be nothing that would replace the mentioned module. I would assume that as the original ASA5500 Series interface module was let go of at the same time as the ASA5500 devices themselves and replaced with ASA5500-X series. I guess that currently you would either need to find the old part from somewhere OR look into replacing the customers ASA with a newer model (that has more ports as default, 6 or 8 depending on model and additional module for even more)
As we are talking about an ASA5510 model then I would consider that its throughput is probably nothing amazing though and you might run into problems if your customer is further expanding its network and forwarding their traffic through the same ASA. To my understanding the new ASA5500-X series by default have more ports so it might be wort looking at a replacement model AND if required getting an additional interface module for that model if needed.
But as I said all of what I said above are written from memory and I might remember wrong as I have had quite a break from these support forums and not sure if I will have time even in the future :)
It seems to me that you will probably find the part ID/numbers/etc for the new ASA5500-X series from this page
http://www.cisco.com/c/en/us/products/collateral/security/asa-5500-x-series-next-generation-firewalls/data-sheet-c78-729807.html
That is if you choose to go with a completely new ASA.
Hope this helps
- Jouni