Hi madmax,
Thanks to the wonders of the Wayback machine … you can see the details of the Amsterdam here.
It sold for a little more than you mentioned (it was $551 in twin in 2005) but it had a 5.6" layer of trizone Dunlop latex with layers of Hollofil fiber in the quilting. This was also made before the current fire regulations.
This should give you a reference point to be able to find a similar mattress out of the options you have available today. I would look for a mattress that has about 6" of Dunlop latex with some type of quilting in a firmness level that is hopefully similar to the firmness level that you had. Post #21 here includes the members of the site that sell latex mattresses online and you can look for mattresses that use a total of about 6" of 100% natural Dunlop and have some kind of quilting in the cover (or you could add a fiber mattress pad to approximate the quilting material) which would probably be as close as you could get.
You could also buy a Dunlop latex core from one of the suppliers listed in post #4 here and then buy a similar cover from the options listed there as well but you may not have any return options if you guess at the firmness incorrectly and need to exchange the layer.
I would keep in mind that either way it still may not be quite the same if the cover or the firmness or design of the latex isn’t the same as the mattress you had.
Phoenix