How to identify plastic or metal adapter?

[aniramca]

For M42-Nikon lens adapter, it is usually made very thin (because of register distance). I am a little concern that it does not have the strength to support long range M42 lenses, which are usually very heavy. Any comments/experience of the adapter buckles/breaks? Are these adapters usually made of metal, or hard plastic-painted black/silver? I could not be too sure that it was made of metal.

[Screamin Scott]

Some are made of brass (chrome plated) & other are blackened aluminum...It seems the ones I have seen that support infinity focus are all made of aluminum & therefore more apt to be damaged whereas the brass ones don't have the optical glass in them. If someone knows who mahes a infinity focus one in brass let me know

[Attila]

As Scott said nobody make plastic adapter.