Dr Ph Martins Spectralite 18K gold on black Rossler envelope.
The Spectralite is opaque and decently rub resistant. I lightly erased the guidelines I drew and didn’t lose any hairlines. The ink doesn’t flow very well on initial upstrokes and hairlines. You also should wipe your nib so that you don’t get stuff dried on it like I did.
I should mail one to myself test how it does with the USPS.