The USA Humanitarian Service Medal (HSM) initially used bronze numbers to indicate multiple qualifications. The first number attached was a 2 to indicate the 2nd award (absence of a device indicated the 1st award). The numbering continued by placing the devices centered on the ribbon bar and/or the medal ribbon. Once 9 was reached and another award authorized, the recipient simply made 10 by attaching a 1 and a 0 next to each other in the center of the ribbon. The number devices were the same size for the ribbon bar, full size medal ribbon, and the miniature medal ribbon, although at least 2 private companies made a miniature device version. While some of the number devices were Government Issue (normally made by His Lordship Industries), the majority were purchased by the recipients and were His Lordship Industries (non-GI), Vanguard, Ira Green, or NS Meyer origin - there are no hallmarks to distinguish them but many collectors can readily identify them. The collector might encounter some of these examples from time to time but the value isn't changed because it's based on the documentation rather than the actual medal.

In the mid-1990's the number devices were replaced by bronze service stars and a transitional period began. Five bronze service stars (or the bronze number 6) were replaced by a silver service star and the normal rules were now followed. The silver star is worn on the wearer's right when further bronze stars are authorized. Depending on the branch of service, they will be placed at different spacing intervals. It is also of note that a style of bronze stars are commercially available which are grouped (2, 3, or 4) on a single device.  By 1 October 1996,  number devices were no longer officially worn by active duty personnel.

No other devices have been authorized to date.

It's also interesting to note that recipients of the HSM often also qualified for a unit award (the Joint Meritorious Unit Award being the most common) of some type if their unit was present (rather than the individual attached to another unit) and may have received a military decoration or a certificate. It's currently believed that there were no recipients of only the HSM for their military service. All documented cases to date have at least one other award authorized.

For further information, please see my blog (http://blogs.ebay.com/tsa-li).  Extensive information is also available online.