How are these "slideable" backpack buckles called? I am asking because I want to get a replacement. The backpack is a "Millican Fraser 32L" but the type of buckle should not be exclusive to this specific backpack.
*** Updated pictures ***
Knowing the model and seller for this backpack was the key. From their website, I found this info about the buckles:
BUCKLES (COMPONENTS) We use aluminium buckles on all our Maverick bags. These are tough enough to withstand an average force of around 122 kgf (kilogram force). The aluminium is anodized, which provides a weather resistant surface without tarnishing, and the anodized trims we use are non-toxic and contain no nickel.
So now we know the buckles are anodized aluminum (or aluminium). By adding that to my search term, I finally started getting some similar hits. Once I found a similar-looking buckle, I was able to find out the term for that shape of buckle.
The second one is a "G hook buckle." Some G hook buckles are flat, and therefore reversible. Others are slightly curved or otherwise shaped, which means they can only be used in one orientation. That type usually has two versions, one where the hook will point to the right, and one where the hook will point to the left. Yours looks like it's probably the flat kind, but I would need to handle it to be sure. You can probably substitute any version that fits the same size of webbing. Here are a few different product listings, purely as examples. It's quite easy to find other product listings for G hook buckles.
The first buckle is not a common style, and there's no consistent naming convention for it, which makes it difficult to find for sale.
A similar buckle is used on the VTAC Scuffle belt:
They don't sell the buckle separately, so you'd have to buy a whole belt. The product description calls it a "smooth front buckle" with an "ingenious interlocking buckle design." "Interlocking belt buckle" as a search term doesn't narrow it down very much.
The British military uses an interlocking quick release belt buckle:
There are other interlocking belt buckles, for example:
As DaveX pointed out, similar types of buckles are used on safety harnesses. In that context, they're called either "mating buckles" or "pass through buckles." Those buckles are difficult to find separately from their harnesses. You can find some of them in bulk quantities on Alibaba, in the Buckles & Adjusters section. Many of the items on this page require a minimum purchase of 100 or even 1000, but there are a few with a minimum of 10, which which makes them somewhat reasonable for repairing a single backpack (usually 10 will cost less than $10, not including shipping). I'm including the product names and links to the listings below; even if the links go dead, the product names may be useful for finding similar items. As you can see, they don't have a consistent naming convention for these buckles.
Climbing Carabiner Tool Customized Black Camping Steel Buckle (this product is only half the entire buckle assembly)
The two parts of this buckle can be named and purchased individually, but it would be tricky to find ones that are compatible with each other in this combination. The piece on the right is a tri-glide
The piece on the left is a webbing reducer
You can easily find the two separate parts using the terms "tri-glide" and "webbing reducer." These types of parts are sized by their interior dimensions (IE, what size of webbing fits into them), not by their exterior dimensions. You would need to know the exterior dimensions of the tri-glide to know if it would fit easily through the larger hole in the webbing reducer. Realistically, you'd need to actually assemble and test multiple combinations to find one that functions well.