Although soapy water will show up leaks better than the purpoe made stuff it is not recommended,,, purely because if left without wiping off it can be corrosive , Funny how leaving a meter door off a box for years is not regarded the same.
Holmslaw have a look on honeywell site you should find something there regards lettering
Problem with the cheap stuff is that it is unreliable. Unfortunately I chucked the bottle after I made my last batch and I can't remember the brand name, but it is easy to recognise. Round bottle, 0.7 litre, half blue half lilac in a wavy pattern. You will also recognise the price, £3.79
But it will not miss small leaks like the cheap stuff.