I believe the NGS maroon leather with gold lettering slipcases started with Jan-Jun 1950 and newer. Prior to 1950 collectors were able to purchase non-lettered slipcases and put their own labels on them.
Yes. If this is true, availability has been curtailed for older dated slipcases with the transfer of sales to Disney. I am now attempting to directly contact manufacturer and printer of the cases to offer to pay extra for them to pull a few dozen aside and date them (to match) with the appropriate years stamps. These would be from 1948 through 1964 with others possible if they agree.
Are you somehow affiliated with National Geographic and, if so, could you direct me to the supplier of the slips or even contact them through official channels for me?
Sorry, I am not - just a collector.
Perhaps you should direct your question(s) to Cathy Hunter who supervises this forum.
You already seem to be abreast of the situation with the gradual lack of convenient access to purchase the slipcases. First the demise of the printed product catalog in 2015, then the shutting down of the ShopNationalGeographic website last spring 2020 where a lot but not all established product was migrated to and absorbed by the ShopDisney store online created a situation where you could not go to a venue to obtain slip cases. The slip cases "slipped through the cracks", but they were still available within new limitations. For example, some people had prior subscriptions to the cases per year. They still received theirs, regardless of the NG and Disney merger upheaval.
Later, around this past late fall 2020, National Geographic Partners LLC transitioned to a new webstore separate from and unaffiliated with ShopDisney/natgeo. Here is the link to the home page, below. Basically NGP is using this to market and supply their back issues (NGM, Traveler, Kids, History, and Little Kids) from current back to 2015, per each title. Then, they have the sub-links for Map Cases, and Slipcases.
direct slip cases link:
Interestingly, they are offering the option to "subscribe" to the slipcases via an automatic billing + shipment each November for that next calendar year. For some reason, 2019 and 2021 cases are available, but not 2020's. However, I did order my 2020 case set in August with this direct number provided by a VP at NGP, John MacKethan -----> 1-888-647-6733.
This may not resolve everything your are seeking, but perhaps it helps, and gives you a contact name + phone # that could yield further information and/or answers to your queries and wish-list.
Well done Scott!