This is a Geissele Reaction Block I picked up awhile back that I haven't ever used. Comes complete with instructions and clamping bolts new in the package. Has a dent, as can be seen, that does not affect function. These are great fixtures for building or working on an AR-15 with a mil-spec receiver extension.
Asking
$120 shipped via USPS Priority; payment by PayPal (F&F or +3%), Venmo or Zelle. Thanks.
Here's a link to Geissele's description for more information:
https://geissele.com/reaction-blockr-ar15-m4-mil-spec-buffer-tube-only.html