According to this I’d say 4.19 but if you can boot it just do uname -r
$ echo 'amber amber-updates amber-security' | xargs -d' ' -Irepository curl -s http://repo.pureos.net/pureos/dists/repository/main/binary-amd64/Packages.xz | xzcat | grep 'Package: linux-image'
Package: linux-image-4.19.0-5-amd64-dbg
Package: linux-image-4.19.0-5-amd64-unsigned
Package: linux-image-4.19.0-5-cloud-amd64-dbg
Package: linux-image-4.19.0-5-cloud-amd64-unsigned
Package: linux-image-4.19.0-5-rt-amd64-dbg
Package: linux-image-4.19.0-5-rt-amd64-unsigned
Package: linux-image-amd64-signed-template
Package: linux-image-amd64
Package: linux-image-amd64-dbg
Package: linux-image-cloud-amd64
Package: linux-image-cloud-amd64-dbg
Package: linux-image-rt-amd64
Package: linux-image-rt-amd64-dbg
Package: linux-image-4.9.0-2-amd64
Package: linux-image-4.9.0-2-rt-amd64
Package: linux-image-4.19.0-5-amd64
Package: linux-image-4.19.0-5-cloud-amd64
Package: linux-image-4.19.0-5-rt-amd64