You are correct 2018 was when most of the Colt cage codes markings on upper receivers, barrels & charging handles started, the specific month is uncertain but probably early in 2018.
Also the QR code of the lower receiver was moved to the magwell area in 2018 and the CR prefix was started in late 2018.
In 2015 cage codes on the barrel and M4 buttstocks was started and QR code above the selector.
For a 2017 build there should be a letter C or no letter on the upper receiver, C on the barrel.
Barrel dates could be a month or several months before the assembly completion date.
2017 was also the year the letter C was mostly missing from the bolt carriers.
As an actual example, I got a new A4 20 inch barrel with Magpul handguard complete upper half conversion kit last month, the upper receiver and charging handle have the cage code, the barrel is marked with a C and no date code.
The bolt carrier had a very lightly marked C, same with the bolt the MPC mark is very hard to see.
I also got a new 6920-EPR (M4 Enhanced Patrol Rifle) conversion kit upper half back in Sep. 2019 no cage codes on the upper receiver & charging handle, cage code on the barrel and 08/19 date. Bolt carrier group had also very light markings.
Colt is always inconsistent in their markings so even one or two components don't match it should be OK.