The archive is huge and it is impossible to summarize its contents briefly. It deals with diplomatic matters such as the arrest of British subjects in Russia/USSR, Russian and Soviet military budgets and military exercises, Russian-Afghan relations, etc., but also contains many reports on internal Russian/Soviet affairs. From the late Imperial period, for example, materials include an evaluation of Peter Stolypin and reports on conditions in various cities of the Russian Empire during the 1905 Revolution and its aftermath. From the revolutionary era there are a number of documents on British assistance to the White forces, German counterfeiting of Russian paper money, and underground activities of the Bolsheviks during the German occupation of Ukraine.
Film A503 -Film A503.1 ... microfilm of actual documents shelved in the microfilm cage in the Lamont basement
Film A503.2-Film A503.3 ... microfilm of document Registers (filed by general document type and date). Registers run only through 1919. See below, "Finding Aids."
Index Film A503.2-A503.5 ...bound paper finding aids for the Foreign Office series.
Using the Finding Aids
There are two separate procedures for using the finding aids...one for materials dated from 1883 through 1905 and another for those dated 1906 and later.
Materials from 1883-1905 ... the researcher should examine the microfilmed Foreign Office registers for the documents first. These registers are handwritten and contain brief descriptions of each document. Documents are organized by general type ("commercial despatches," etc.) and by year (and sometimes month) of arrival in London. The researcher should note the category and date of the documents s/he wishes to examine. With these notes, s/he should then consult the bound paper finding aid (Index 503.2) which will show which microfilm reel documents of a given year and type are on. The system is clunky and as far as I have been able to determine, there is no subject index.
Materials from 1906 and later...the bound paper finding aids for 1906 and later (Index Film A503.3-A503.5) are organized chronologically, by general document type, and sometimes by consular provenance (St. Petersburg consulate, Warsaw, etc.). Thankfully the bound finding aids describe each document and also show what microfilm reels documents are on (they also show the volume number and page number in bound Foreign Office document series sitting somewhere in London, but that's irrelevant for the Harvard researcher). In short, go straight to the bound paper finding aids. There is no need to consult registers (which don't exist anyway after 1919).