I saw something similar handled by the late Bo Hansson (of Rauna of Sweden and Opus 3 records fame)... he used these (plastic) adapters with his Telefunken M15A open-reel tape recorder... look at right spool here below.
Apparently not simply an higher, oversized NAB adapter... but - maybe - someway looking like a 2 inches tape-reel adapter, cannot say 100% from the pix.
Almost impossible to find - at least I never found it anywhere on the WEB - I finally had one pair bespoke made for my personal use: hand made/machined in solid aluminium, which I'll have anodized in the very next hours.
A labour of love.
This way, I'll be both able to use my NAB-cores tapes in pancake or the classic, complete tape reels, always using the superb Telefunken's DIN reels holders.