Aerospace Material Specification 3411 is a silver-brazing flux in a paste form.
This black flux meets Federal Spec O-F-499 and AWS FB-3C.
AMS 3411 is engineered for use in high temperatures may be used with silver or other brazing alloys up to 1800℉ for extended or uneven heating; specification is also well performing in low temperatures. Ideal for welding stainless steel or wherever higher temperatures or extended heat duration. Not recommended for use with alumminum, magnesium, or titanium.
Further reading: brazing flux SDS.
Further reading: BAg19 brazing filler metal.