There is a lot of mis-information in this thread. Marine swords have always been one of the least, if not THE least, understood series of US military swords. I have done considerable research, primarily in the National Archives, to try to clarify the history of these swords. Man-at-Arms recently published the results of my research on the Marine M1859 NCO sword and its variants in its Oct 2018 edition, and I will probably do another article on Marine musicians swords in the near future.
As for the sword at the top of this thread. it is quite possibly an early CW Marine M1859 NCO sword.
The Uniform reg of 1859 was not very specific on the design of the NCO sword. It merely stated it should be "same as US Infantry". The Commandant later issued some clarification to the Quartermaster as to what was meant, i.e. "It is intended to be of the same pattern as that adopted for officers, except the hilt is brass instead of gilt." Actually, there were a number of differences between the NCO and officers' versions. Sergeants' swords had plain brass hilts and scabbard mounts, whereas officers' hilts and scabbard mounts were gilt. The grips on sergeants' swords were wrapped with leather, whereas officers' grips were usually covered with sharkskin. Officers' swords had etched blades, while early Marine sergeants' blades were polished bright with no etching. Finally, and the most visibly obvious difference, the scabbards of sergeants' swords had only two mounts, a top mount with a frog stud and a scabbard drag, whereas officers' scabbards bore three mounts, i.e. a throat and middle mount fitted with carrying rings, and a drag. The sword at the top of this thread has leather wrap, the blade appears unetched, and there is no sign of gilt on the hilt. The markings are also the same as those found on the Horstmann-made NCO swords delivered in 1859. With no scabbard, however, it is hard to say with absolute certitude that this isn't a bargain-basement version M1850 foot officer sword made with cheaper leather grip covering, no etching, and thin gilt which has worn off.
A couple posts question whether Horstmann had contracts to produce Marine swords. In fact, almost all Marine enlisted swords produced in the second half of the 19th century were made by Horstmann. They had the initial contract to produce M1859 swords. In 1860 they lost the contract to Ames but regained it in 1861. In 1862 Bent and Bush won the contract and retained it for the rest of the CW years. B&B, however, was not sword maker, and it is believed it sub-contracted the sword portion of their larger contract to provide military goods to the Corps to Horstmann. There are no known examples of Marine swords with the B&B mark. On the other hand, with the exception of one probable Ames example, all known CW-era Marine swords are marked to Horstmann. In the post-CW era, Horstmann regained the contract to provided enlisted swords to the Marine Corps in 1869 and, with the exception of two years, retained it for the rest of the century.
One of the above post shows a picture of the Horstmann M1840 NCO sword with the turned-down rear counterguard and states it was the CW Marine NCO sword.. This is a common mistake which has been around for years and even appears in some respected reference works, e.g. Col Rankins "Arms of the Sea Services". This is probably due mainly to the ambiguity of the Uniform Reg of 1859 language noted above. In fact it is simply the Horstmann version of the M1840 NCO swords which they marketed to Army and militia elements. Just as steel scabbards were unique to Emerson & Silver-made M1840s, so was the turned-down rear counterguard unique to Horstmann. There was, however, a Marine connection. Like modern collectors, Horstmann and the Marine Assistant Quartermaster in Philadelphia were confused by the wording in the Uniform Regulation of 1859 and thought it meant Marine Sergeants would carry the Army M1840 NCO sword, and so initially sent one of Horstmann's M1840s to USMC HQ in Washington as a sample. This is when the Commandant sent his clarification. HQ must have liked the M1840 sample, however because a few days later they informed Asst QM Philadelphia ""The sword first sent by you has been adopted for the musicians of the Corps, and the one the same pattern of the officer's sword, with brass hilt to be worn with a frog, has been adopted for the sergeants." Thus some Horstman M1840 NCO swords were used by the Marines, but by musicians, not Sergeants. The majority of such swords, however, were used by Army/militia forces, not Marines. There is no way of differentiating which is which.
Sorry for the long-winded reply; just wanted to use the opportunity to correct some widespread misunderstanding about these swords.