Here is my plated vintage buckle. With "Plaque G" on the back as mentioned. All original.
In addition to what have been mentioned be aware of the finish. Here's a compare shot of a genuine and a fake s/s 16mm big logo one. The logo and edges are much more sharp and detailed on the original. On the fakes they are smooth, almost like overpolished. It's the real deal on the strap.
And here are some different styles😀 It's hard to make a firm set of rules for the writing on the back. All depends on the manufacture or in which country they were produced...and what year. Some have sun markings, some have Omega markings, some just acerinoix and swiss + GM or depose and some with mixed, some with different spacing in the writing and some differs depending on their size. The gold ones I don't know much about besides the plated above. I'm a steel kinda of guy😀