Some people have got it here.
http://www.mchardys.co.uk/prodet.php...28&rowpos=1628
I found some Gudebrod braided mono, also no longer made, on eBay.
Most mid to high end lines now have very good loops but they can wear out
I will not use a nail knot to connect a leader butt on my heavy weight gear. I have seen anglers who insisted this was sufficient watch as yellowfin tuna and yellowtail just strip these off the end of a fly line. I do use this type of connection on much of my freshwater setups though. The nail knot connection works well across a wide range of fishing conditions
A properly constructed and properly attached braided mono loop will be stronger than the fly line. Home made double catch loops are much stronger than most of the commercially available loops which have a single glued catch. I have had a couple of fly lines break before the loop failed.
Several good suggestions in this thread for alternatives to braided loops. They all have their place.