These adaptors and the practice of refilling 1 lb cylinders is against the law in Ontario as it violates the Propane Storage and Handling Act as outlined in this TSSA bulletin:
https://www.tssa.org/corplibrary/ArticleFile.asp?Instance=136&ID=6D4CD6C35AD311E0B7F95D889551D742
That said, there are a lot of things that are illegal that people still do from time to time and I am no different. I have in fact refilled hundreds of 1 lb cylinders and with no sign of law enforcement showing up at my door. It is dangerous however and need to be done with due care and caution.
Here's my notes which are in addition to all the other safety measures already mentioned:
1) Don't overfill or the relief will vent when they warm up. The empty bottle weighs about 14 oz and you should only put 16 oz (1lb) of propane in them. If you fill them full with no head space, they will vent wherever you have them stored with possible catastrophic results. It is hard to predict how long it takes to fill a bottle; it depend on the temperature of the the tank and the bottle and
semi-full bottles fill faster than empty ones (they do not need to be bled down as someone else suggested).
2) Don't get too greedy. Fill the 1 lb'rs no more than 5 times and toss them. I mark mine with a magic marker to keep track. I have had zero leakers after 5 fills but a few after that.
3) Install the bronze cylinder caps on the refilled bottles if you can find them; second line of defense against leakers.
4) Keep the big cylinder at room temperature and put the empty bottles in the freezer for an hour or so before refilling. This will allow liquid transfer without having to bleed the relief vavle on the small bottle (this eliminates some of the danger).
5) Burping them by pulling on the relief valve stem to get them extra full is a bad idea as the reliefs don't always re-seat properly and can leak. If you have an underfill, just put it back in the freezer for a few minutes and top it up again. If you have an overfill, let the small cylinder warm up and it can then be transferred back into the big one with the big one in the upright position (the transfer is all based on temperature....).
DISCLAIMER: this is only my experience and I am not claiming this is legal or sensible.....use caution and your own discretion if you really want to do this....