This is PU sealer and is known as such in the car body trade. If bought from a supplier to body shop trade [there is nearly always a couple of these in every big town either part of a chain or independent], these tubes are unmarked and sold in either black or white pigmented. They usually cost about £5 each.
The seal is also sold with trade names, the most famous is “tiger seal”. It is just the same but will cost about £12. The seal is also now sold in “B&Q” for outside sealing purposes and again will cost £10+. This may be adulterated.
This seal has a short shelf life even unopened. When opened, the remains of the tube should be used within a couple of weeks or it will not squeeze out of the tube. It is wonderful stuff, not just joining rubber but as a very superior filler keeping moisture out of inaccessible places such as where the chassis kicks up above the back axle where the shock absorbers are fitted which rots out very easily.