This specimen comes from the Cenomanian of Nammoura, Lebanon.

See: Tachypleus syriacus (Woodward)—a sexually dimorphic Cretaceous crown limulid reveals underestimated horseshoe crab divergence times

I had only just learned about sexual dimorphism in modern Tachypleus last Thursday when I saw large males and females displayed on a wall in Kampung Ayer, Brunei. I then remembered the two indentations visible in this Lebanese specimen, something I have owned for many years now. A search revealed the 2015 paper by Lamsdell and McKenzie.

There's a new paper on the limbs by Bicknell et al. 2019. On the appendicular anatomy of the xiphosurid Tachypleus syriacus and the evolution of fossil horseshoe crab appendages.