From Peterson Pipe Project...doesnt explain the missing marks but is interesting. The missing marks may make your pipe quite unique.
Silver Band Dating
Silver hallmarks are placed on the silver after an assay office, in Peterson's case, the Dublin Assay Office, has verified that the silver content is indeed sterling, in other words 925 parts of silver per 1000 parts of the metal. The silver hallmarks on Peterson pipes are a group of three marks, each in an escutcheon; the first is a seated Hibernia denoting Dublin Ireland, the second is a harp denoting the silver fineness, and the third is a letter denoting the year. The style of letter and the shape of the escutcheon the letter is in, will determine the year in which the assay office stamped the metal band and not necessarily the year the pipe was made. Peterson orders these bands by the thousands and sends them to the assay office for hallmarking. The assay office will stamp the date of the year in which they received the bands and it may be a year or two or three before Peterson's employees happen to place one of these bands on a pipe though generally the bands are placed on a pipe in the year they were stamped.