Tony,
You're correct that the 2266 number on the bonnet flange is the construction number; these ran from 1 to 20082 for all berlinas. The chassis numbers for 1st series berlinas started at 38-1001 but the numbering wasn't continuous. See the attached pdf for the complete Aprilia numbering system. The page from the manual shows where the relevant numbers are found.
The chassis number is only the manufactures plate and, as you have found, stamped on the firewall (very early cars didn't have the chassis number stamped on the bonnet flange). You won't find either of the numbers stamped on any other components, a few have the part number and most will have Lancia & the year stamped on them somewhere. Original parts in your car should have Lancia 1937 inside a little oval.
In the parts books the construction number is used to show when changes were made during production. I've got a bit more info on my page here:
http://www.narrywoolan.com.au/home/1st_series_changes.htmlHow the numbers got mixed up in the logbook is a mystery though!
Noel