The 75mm barrel on my E is a Fine Molds brass barrel for the "Hetzer" tank destroyer.
I think it was Fine Molds...I chucked the package it came in!!
Jordi Rubio or Eduard probably have several types of 75mm barrels for sale.
The barrel I used is substantially longer than the kit barrel, but I liked it that way.
By the way, the 128mm barrel on my E is the plastic one from the Maus kit.
Initially, I had planned to use a metal barrel, but that would have required some precision, large-scale drilling of the Mori Artisan resin mantlet, which I didn't feel like doing at the time.
Aside from the lack of rifling in the tube, the Maus' 128mm barrel is fine!