Whilst sorting out car items in my garage, I today came across a box of spare m/c horns, some of which I have examined & measured, so offer the following comments.
I do not recognise the item on Ebay.
The 1930’s Klaxon horn fitted behind the Scott Squirrel casting would carry a brass plate “KLAXON LTD, 38, BLANDFORD ST., LONDON W.1.” , with a serial number, possibly, N ????? . The housing behind the front plate mounting is very deep at 2 inch.
An original Scott Squirrel casting will fit over a LUCAS ALTETTE horn with minimal modification, as Dave Bushell has indicated.
Assuming the new SOC Scott squirrel castings will also fit over a 4 1/2 inch diam. horn, there should be no problem.
If you are serious in looking for an original Klaxon, I will happily photograph it and email it to you, my email address is in YOWL,