Yep the Jaycar one is on ebay too but I've just received the Gaslock pen and it works fine on my 8.5kg bottles and a reserve 2kg bottle I carry. I settled on the pen one as most suitable for my Jayco situation where the 8.5kg bottle sits in the typical cara type holder on the drawbar. As I surmised looking at the different types, the pen allows me to even check within the bolt together gap between the upper steel strap (apart from the actual bolt itself) whereas that wouldn't be the case with the others. Essentially I want to be able to check the bottle in situ, anytime when I'm all packed up, not crouching under the bed extension and needing hot water off the stove.
Your situation may be different but the pen ticks all the boxes for me, unless your choice could also check the water level in my poly tank (the pen can't even detect water level in a stainless steel kettle so it's LPG only)