They may not posses a
Made In USA label, but they're confirmed to be US made.
Regardless, they're top quality IME (3M labeled in my case), and arguably the best available from any manufacturer. Even vs. US made Global Specialties (i.e. PB104) when pushing the frequency limits (otherwise I couldn't tell). For disclosure, mine where manufactured in the mid to late '80's.
But buying directly from ASP in the US at least is a no-brainer it seems IMHO (cheaper than an equivalent from Global Specialties non-Economy version, which are still purportedly US made). Suspect that this may also be the case in the UK/EU. Under the 3M label OTOH, ...
Cost wise, it's nasty with a 3M label slapped on it either way.
Worst case, I wouldn't go less than Taiwanese made Wisher (also decent stuff; bought a year or so ago from a RadioShack that was closing). Not gold plated though, so I'm not sure on long-term reliability vs. the gold plated ASP/3M I'm familiar with (IIRC, Global Specialties is nickel plated).
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All this said, I actually prefer to use inexpensive protoboards (i.e. dual sided PTH pad-per-hole + SMD IC break-out boards) and solder everything. Even if it's truly not the case, I do this as it makes me more confident in the electrical connections vs. solderless breadboards, especially if you're working above ~10MHz. FWIW, I find testing a filter is usually fine within the previously mentioned ~10Hz max frequency (i.e. quick & dirty method).
Oh, and I can use this method using strictly SMD parts (i.e. 0805 works rather well on the dual sided PTH protoboards IME). Generally speaking, cheaper too.
YMMV of course.