Have no idea what they use but I have had good success over the years on some remingtons with the gold inlay
Have used a Forester gold inlay kit
Cheers
Product Description
Add a touch of elegance to engraved firearms
This handy kit fills engraving and lettering. In addition to highlighting custom engraving jobs, it gives production guns a personalized appearance. It’s a great way to enhance the value of either new or used guns. Easy to use – just follow the simple instructions. Although the kit is designed for gun work, it’s also ideal for other hobby areas including model railroad engines. Kit includes enough material for five to ten guns. Gold color is actually a fine bronze powder. Not intended for use on parkerized or ferritic carbonitrided surfaces.