OK - just checked my touring bike which is a 2006 vintage and other points you might like to look for are "ROBERTS" stamped on the bottom bracket shell both sides, also on the top of the seat stays. I don't know how universal they are but am sure other owners will confirm or otherwise. If you pm me contact details I can send you pictures as I can't be bothered to upload anything here.
My frame number is underneath the bottom bracket and is 6 digits - 2 for year, 2 for month and 2 for sequence of bike in the month. Our tandem is newer but still the same format for the number.
If it has original Roberts forks the number will probably also be on the steerer tube (based on two out of a sample of two) but I appreciate that isn't readily accessible as a potential buyer.
In 2006, in terms of "decals" you could choose how many or how few (aside from the head tube badge) and the colour etc because they were painted on, so mine is pretty minimalist with it just on the down tube, but others have more. The font was changed later so our tandem is different. Again can send photos of both.
Re the components, essentially you could choose what you wanted, also depending on the age it could have been replaced so not sure that it is a very good guide.
Check out this page
www.robertscycles.com/ and the linked page lcc.org.uk//articles/roberts-cycles. Note if you can get the frame number it may be possible to get more information.
Hope this helps