
That is insulator hard card board. Sometimes waxed, sometimes not. If your test finds asbestos in it I would be amazed. Pictured below is what asbestos that was used in the Victor and RCA Electrolas looks like. It is light grey in color and very fibrous and very easily torn. Thus if it was out from under an amp it was covered with tin to keep from damaging it. It was never used as an electrical insulator. It was used solely for heat protection to the cabinet from power resistors and the ballast tube. If you encounter the real asbestos and are that afraid of it, do't be. Just don't sniff it or lick it. You you are that concerned you can buy a thick white ceramic paper on line that looks very similar to the asbestos. That way you can replace it and retain the same look. See photos below.
If it makes you feel better to have it tested I just hope it does not cost you much.

Best regards,
Carlton Smith
Indianapolis, IN
electrolaman 64