Tom:
The gun you mention is a 4" Blakely Rifled gun. It is further described as a 18 pounder, referred to as a type 7 Blakely. This type number 7, was named as such by Warren Ripley in his work to catalog the various Blakey models of cannon. The rifling of these guns had six groves. There are two surviving examples of this gun, both at the US Navy Yard in Washington DC.

Although this gun has been considered small as a siege gun and too large for field service, Warren Ripley suggests this model has been considered for use in a dual role, field service and for siege warfare. One of the surviving examples was captured in August, 1863, near Fort Fisher NC without a gun carriage. The registry number of this gun is #136 made in 1862. The other example was found in July, 1972 after one hundred years underwater. It was found in the Roanoke River at Fort Branch, near Hamilton NC. This gun was recovered still attached to its steel gun carriage with a registry number of 138 presumed to also have been made in 1862. Also recovered was some Blakely shells that fit the 4" (20 pounder) Blakely Rifle. The source of information is the book "Field Artillery Weapons of the Civil War", revised edition of 1988, ISBN #0874133467. The authors are James C. Hazlett, Edwin Olmstead and M. Hume Parks.

Based on the information you provided and that in the book "Field Artillery Weapons", it can be presumed that General Pendleton must have been successful in refusing the Blakelys for service with the Army of Northern Virginia as none appear to have been at Gettysburg in July 1863. His letter, as you mention, was dated June 1863 in which he was trying to find a home for them. Presumely, they were sent to North Carolina for coastal artillery duty where one was promptly was lost near Fort Fisher.
Hope this helps
Ron

My souWashington