Does anyone know what the silver plating is on the older diana grade browning o&u shotgun receivers?

In a word no, but I believe it is a 'French Gray' finish. It is supposedly done with phosphoric acid and then given a clear lacquer coating. I am not sure how Browning actually does it.
C.G.B.

You may have come across this information which was/is on the Browning website. There is no indication of when this process was first applied to Browning shotguns so may not answer your question in regards your early model Browning.

From the Browning website;

What is Salt Bath Ferritic Nitrocarburizing?
Browning shotguns with a "Silver Nitride" receiver utilize a proprietary process that is known among those who create metal finishes as salt bath ferritic nitrocarburizing. Over and unders made at our Miroku factory use a product with a trade name of Tufftride. The process leaves the receiver and other steel components with a bright, silver color while at the same time improving the finish and surface integrity in three ways:.

Greatly improving scuffing resistance
Improving fatigue properties
Increasing corrosion resistance.
One major additional benefit is that while the process is improving the finish it induces very little shape distortion during the hardening process. This is very important to a finely fitted Browning over and under shotgun.

Getting Technical - Controlled Liquid Ionic Nitriding (CLIN)
A more technical description is provided by the company who owns the process and can be helpful if you are looking for more a more detailed answer. Controlled Liquid Ionic Nitriding (CLIN) is the family name of different nitriding / nitrocarburizing processes which are well known under various tradenames. CLIN (controlled liquid ionic nitriding) treatment can be used in a temperature range from 480 °C to 630 °C and enriches the surface of ferrous materials with nitrogen as well as small amounts of carbon. CLIN salt bath nitriding greatly enhances properties like:

Wear resistance
Fatigue strength
Corrosion protection
Cosmetic appeal
Compared to coating or heat treatment processes CLIN salt bath nitriding offers numerous advantages.

Thank you for this information.

quote:

Over and unders made at our Miroku factory use a product with a trade name of Tufftride.

This seems to indicate that the process described only applies to the Japanese-made Brownings and may or may not be similar to a Belgian Diana Grade.

But I have no information which would otherwise be helpful.