In my experience, pre wound bobbins that don't specify are usually a 60 wt. Typically, prewound bobbins are geared to machine embroidery, specifically commercial machine embroiderers. So because virtually no stress is put on the bobbin thread in that particular application and the commercial machine embroiderers want to be able to get a lot of stitchouts between bobbin changes the thread is usually a lighter weight and often only a two ply. I checked Guterman's website and it isn't even specified there. I wouldn't stress too much over the weight of your thread in the bobbin. With sufficient quilting it should hold up well enough. If you are truly concerned over it, don't use them, use a bobbin where you know what weight and ply thread you have.
Edited to add, the only Gutermann prewounds I could find were polyester so you may have something different.
Here is a link
https://www.guetermann.com/shop/en/v...&type=consumer