(part of) regulation is practically there as pre-collection for femme/homme shops. sort of subcategory though. what comes in the same shopper and arrives between the last delivery and the first delivery. maybe CDH too.
anyway will try searching for Y's for that period.
That's exactly how I was able to find all the pre-collection letters since 2013! By some miracle, the product codes were listed every season on the Regulation website, which I accessed through the Wayback Machine. What I found is that, every season, Regulation used two different letters: the main season letter and an inter-seasonal letter.
Unfortunately, Regulation was launched for the Spring/Summer 2013 collection, so I can't use this to find the inter-seasonal letters before that. To complicate things further, I've been finding 2 letters per inter-seasonal period until Spring/Summer 2015. It seems like the inter-seasonal letter used by Regulation differed from the pre-collection letter until then.
For example, for Spring/Summer 2014, the main letter was "G", while Regulation also used the letter "I" that season. However, I found evidence that "M" was used for the Spring/Summer 2014 pre-collection. Similarly, for Autumn/Winter 2014-15, the main letter was "F", while Regulation also used the letter "H", although "V" seems to have been used for the pre-collection. Then, for Spring/Summer 2015, the main letter was "U", while Regulation also used the letter "J", although "E" seems to have been used for the pre-collection.
After Spring/Summer 2015, the secondary/non-main letter used by Regulation matches the pre-collection letter. And since the pre-collections seemingly started in Spring/Summer 2008... I'm trying to figure out the period between 2008 and 2015.