Uploaded from WIKIA, which uses CC-BY-SA license, so this should be repostable.  File by Lord ironfield.
<a href="//commons.wikimedia.org/wiki/User:JMvanDijk" title="User:JMvanDijk">JMvanDijk</a>
CC BY-SA 4.0
<p><span class="int-own-work" lang="en">Own work</span>
</p><p>see <a rel="nofollow" class="external free" href="http://downtonabbey.wikia.com/wiki/File:Grantham_Arms.png">http://downtonabbey.wikia.com/wiki/File:Grantham_Arms.png</a>
</p>
The coat of arm can be seen in season 2, episode 8, time 26:30 (grantham arms).