Okay, I've figured out what's going on here.
That punch you aptly describe as a portcullis, is actually a Birmingham assay office anchor, that has worn in a particular way to make it look like it has three vertical bars. I'm sure if you look at it closely with a magnifying glass and roll it back and forth in strong light you should see the shape of an anchor. Here's one that's done something similar and starting to look like a portcullis too.
And if the date letter is "s" that would make it for the year 1917 -18. By this time, the silver mounts for Howell walking sticks were being made in Birmingham, rather than London, and the silver work assayed in the city where it was made even though the sticks continued to be made in London.
Hallmark punches subjected to knocks and wear in a soft metal like silver can do strange things to the marks which can become oddly misshapen.
So we have an Edwardian ebony walking stick, fully marked for J. Howell, dated 1917 with a silver collar band. In the current market it would sell at auction for £50 £90. It therefore has a full retail value of £180 (if you saw it for sale in an antique shop, say). This is also the replacement value for insurance purposes.
I do hope this helps!
Howell advert from the time.