Kiehl's musk fan here and this one had been buggin me for a while so... after 2 emails and 2 visits to the Kieh's brick & mortar on Michigan Ave in Chicago, I've got the following consistent representations:
1. The 160th Anniversary Limited Edition Musk Eau de Toilette Spray and the Original Musk Blend No. 1 EDT are the same/identical fragrance.
2. There is no marketed essence oil roll-on or 'splash' form of the 160th Anniversary Limited Edition Musk.
3. The .23oz Musk Essence Oil Roller Ball Applicator oil (aka “1921”) and the 1oz./.5oz/.25oz ‘splash’ vials with the Red/Blue ribbon (also prominent on the 160th Anniversary Limited Edition Musk Eau de Toilette Spray bottle) are the same/identical essence oil.
4. The EDT and the essence oils are the same fragrance aside from the essential oil content or concentration.
5. 1921 being the year that the musk essence was purportedly first created and introduced.
While I've never smelled the 160th Anniversary Limited Edition EDT I have multiple bottles of the others and have no reason to doubt the representations per above. If anyone knows anything else, I'd be interested to hear it.