The last time it was serviced was 1979. I think the gasket was glued in. It would not come apart 'till this year. The gasket rotted enough that it tore in half, leaving half on the watch and half on the caseback.
Right after that I took it to the watchmaker for a service.
And you're right. That goo is very messy and hard to remove.
Bought the watch new in '69. Still have box & papers.