Weren't nothing "custom" about them. They were made by SKB in Japan and imported by Ithaca for a number of years. They were in the same price bracket as the Browning Citori and Winchester 101 also made in Japan.
The Model 500 was the standard version and the 600 was a wee bit more deluxe with as I recall a silver looking receiver rather than blued and maybe a little better wood.
They were notorious for beating up the face especially in the 12 ga. I had a 20 ga. Model 500 once and even it beat up my face a bit more than I liked so I traded it off. The 28 ga and .410 bore having so little recoil are really nice. The 20 ga. with regular skeet level loads is OK. It is really no better or worse gun than a Win. 101 or Browning Citori both of which carry a little better resale value even if only slightly so.
GB