This cabinet looks like one of a production run done several years ago of "new" SP-600-style cabinets that was put together by a member of the BA reflector at theporch.com.
While these were made by the same company that built the original cabinets for Hammarlund, they no longer had the square vent dies from the original, and the cost to build an actual reproduction would have been prohibitively expensive.
These are nice cabinets (I still have one) .. but not SP-600 original, and not $350 worth at any stretch.
You're correct. Cal Eustaquio, from CA, worked a deal with Premier Metal in 2000(?) to build about 200 of these cabinets. Cal solicited interest on several reflectors to gather interest and funds for the project. I believe the cabinet sold for around $190 each and, with shipping direct to your location, I believe the total came to around $218. The first batch was single boxed and some suffered damage. The next batch was double boxed several weeks later, and, I believe, all were received in fine shape. I have two here and both came through in fine shape. They are very nice cabinets.