• ALGOL 60 Publication and Compiler Status: ALGOL 60 specification published in May 1960. People were expected to write their own compiler instead of getting a source code of a corresponding compiler like today. In June 1962, the paper "The Replies to the AB14 Questionnaire" listed implementation details of about 21 compilers (unclear if some are dialects or similar languages). In 1963, there was a list of 32 Algol compilers/versions.
• Compiler Writing in the 1960s: Compiler writing was a hot research topic. In the 1960s, all the techniques we take for granted today had not been invented yet. For example, Knuth invented LR parsing in 1965 and algorithms for optimal code generation started appearing in 1970. The 1960s was the period of the "Cambrian explosion" for programming languages.
• Implementation Challenges: Implementors had to deal with unknowns of writing a compiler and use systems with limited memory (measured in tens of kilobytes), interact through punched card or punched tape, or have the luxury of teletype input/output. Fourteen of the implementations considered themselves a "true subset" meaning everything implemented was as per the specification. Compilers for earlier languages might have had an advantage as the language didn't support hard-to-implement features.
• Book on ALGOL 60 Implementation: In 1964, the book "ALGOL 60 Implementation" was published, which gave a major boost to compiler implementation know-how.
• Implementation Time Plot: The plot shows the number of compilers with a given implementation time. The median implementation effort is 2 man-years. The 0.25 man-year implementation looks like a port, the 10 man-year implementation is for a full implementation with extensions, and the 80 man-year implementation was by SDC for a range of JOVIAL compilers targeting five different hardware platforms.
• Implementors of Algol Compilers: It's not possible to say if the implementors of Algol compilers were different from those of other languages. ALGOL 60 was created by a committee of computing academics and like-minded people, while Fortran was dominated by IBM and COBOL was designed for business people.