Most bicycle manufacturers start building the new models around September. This allows them to build-up inventory and compensate for trans-oceanic and distribution channel shipping times, yet still have the bicycles in shops for the lucrative Christmas season. Also, the first of the new models have to be available in time for display at the autumn trade shows, where distributors and dealers place orders. The big European shows were in Paris and Milan, and typically took place in early October and November, respectively. This also allowed them to capitalize of any successess at the World Championships, which typically take during the last week of August or First of September, while they're still fresh in the mind of distributors and dealers.

In order for the bicycle companies to start building the new models during the last 1/3 of the year, component manufacturers have to produce their new models even before that. It's not unusal to find mid-year component date codes that actually represent the upcoming model year.

Consequently, while New 105 had a 1987 model year debut, it's quite common to find them with mid 1986 date codes on frames that were built in late 1986. It's not unlike the automobile industry, which routinely has the new models out by now. In fact, I understand that a number of automobile manufacturers introduced some their 2022 models as early as this past spring!