Are you positive of the date? If so, it is not a penny - it is a half penny.
In 1775 the penny was small and made from silver.
I don't think any pennies were minted in 1775, and the earlier (1772) and later (1776) were inscribed 'GEORGIVS III DEI GRATIA'.
In 1797 the first copper penny was struck - and it was so big it was nicknamed the cartwheel. It was inscribed 'GEORGIUS III D G REX'.
This particular penny was struck until 1806/1807 - but kept the 1797 year.
The half penny of 1775 is copper, 30 mm approx diameter, is inscribed 'GEORGIVS III REX' on the face with a bust of George III.
On the reverse is Britannia with the date.
This is the half penny:
http://i16.tinypic.com/48mp0ua.jpg
Britannia is on the reverse and facing left, holding a trident and olive branch. It is stamped Britannia 1775.
I have found several 1775 half pennies that have sold any eBay - a good indicator of value:
http://search-completed.ebay.co.uk/search/sear ch.dll?GetResult&sacat=-1&ftrt=1&coaction=comp are&fsop=1&ga10244=10425&sadis=200&from=R10&sa rgn=-1&sofocus=so&copagenum=1&fpos=Postcode&sa prclo=&fis=2&maxrecordsreturned=300&sbrftog=1& frpp=50&satitle=1775+half+penny&saprchi=&coent rypage=search&ftrv=1&guest=1
Hope this has helped.