I have heard that Shimano releases an updated or redesigned component line about every four years. The old technology then passes down to the less expensive product line. If this is true, theoreticaly a newer Ultegra component would be a better buy and possibly of better quality than an older Dura-Ace. So my thought is, can you reliably date a component that has no serial numbers or manufacture date.
All Shimano components have a series number (not serial number) embossed or engraved in the component. While it is possible that the numbers may be worn off or scratched over on older components, the theory you are working under seems to indicate that you would be looking for newer, lower level components, rather than older higher end stuff.
Check the back of the inside front derailleur plate for a number like FD-6700, for example, (which would indicate current Ultegra). There is a number on each component, with the first 2 letters indicating the type of component, and the number indicating the series.
- CN = Chain
- ST = Shifter
- BR = Brake
- FD = Front derailleur
- RD = Rear derailleur
- HB = Hub
- WH = wheel
- BL = Brake lever
- CR = Chain ring
- CS = crank set
The higher the number the higher the component level. Current 11 speed Dura Ace is 9000 series. Last year's 10 speed Dura Ace was 7900. The series prior was 7800.
For Ultegra, the upcoming, not yet released group for Ultegra will likely be 8000. The current is 6700 for most components. 6600 was the prior group, and 6500 before that.
If the 3rd number is a 5, as in 6650, (ususally only noted on the crank) then the group came with a compact crank.
If the 4th number is not zero, it is either a revision, in the case of CN-6501 or 6502, or designed for a triple crank. This will be noted on all parts, as shifters and derailleurs also must change for a triple geared crank, in most cases.
If the 3rd number is a 7, i.e. FD-6770 the part was designed for electronic shifting. This will be on derailleurs, batteries, wiring, and shift levers. If you find a number on any single component, it can be used to date the whole group set.
I hope that is useful.