I've been thinking about this on and off for the last hour. After trying out dozens of sentences, I am no longer sure what I say, but I think I might say:There's a silent b at the end of 'lamb'. I think 'on' is acceptable.
Stick an extra t on to the end of 'set' if you are writing about a badger's home. I think I'd accept 'at', or just 'on'.
If you use the word "untoward", don't add an "s" at the end of the word. I'd accept 'to' and 'on to', but not just 'on'.
I know, I know. There is no logic there - and I might give a different response tomorrow.