Below are all of the known patents for Howe-Orme instruments. 

The first, patent number 508,858, dated November 14, 1893, is a Utility Patent (i.e., a patent specifying some functional feature of an invention) assigned to James S. Back, who is listed as "assignor of one-half" to George Lewis Orme (i.e., who confers equal ownership on Orme).  This patent describes several innovations to the design of a guitar or other similar instruments, including a mandolin.  Among these are a "raised longitudinal belly ridge," that characteristic Howe-Orme feature.  This is really the only one of the innovations mentioned in this patent application that appears to have found its way into actual instruments, as far as I know.  It is the first reference to the "cylinder top" design and specifically mentions its applicability to mandolins.

Next is another Utility Patent, number 538,205, issued on April 23, 1895.  This patent describes "a ready means of adjusting detachable necks of guitars, banjos, mandolins and other stringed instruments of a similar nature."  As far as I know, this design feature was implemented only on guitars and was very successful.  

The next set of documents are three Design Patents.  These specify the appearance rather than the function of an invention.  They were assigned consecutive patent numbers and are all dated August 24, 1897, the patent date appearing on Howe-Orme mandolin labels.  Each portrays an application of the "longitudinal central swell" to a different instrument. 

The first in the numerical sequence (i.e., patent number D27,559) shows the cylinder top design applied to a bowl-back Neapolitan-style mandolin (absent its usual top cant, of course). 

Next in the sequence of three Design Patents is D27,560, the one depicting the guitar-shaped Howe-Orme mandolin body.  The illustration shows a typical Howe-Orme instrument of this type, with single-lobe headstock, geared tuners, scalloped tailpiece cover, and a floating bridge with decorative scoops near its ends.  The only feature associated with these instruments that is not depicted is the pickguard with the ornate monogram inlay.  Both of these Design Patents applicable to mandolins bear the name of Edward F. Howe as the only patent holder. 

In contrast, the third design patent, D27,561, bears only the name of James S. Back and describes a guitar with the central hump.  The instrument has a fixed bridge and strings are attached with pins as on a standard flat top guitar.