ABB S91 series plug-in circuit breaker, 3kA, available in 6, 10, 16, 20, 25, 32 and 40 amp ratings. Designed to replace porcelain fuses on older switchboards which do not have DIN rail mountings. Although these circuit breakers can be fitted directly to a lot of the older porcelain bases, to comply with current standards the matching plastic safety base (available separately) should be replaced as well.
Tip: It is often better value to consider a surface enclosure (or even a complete new distribution board) and modern circuit breakers (much higher protection rating) when replacing porcelain fuses as it often becomes a case of pouring good money after bad. If you are in doubt please send me a photo of your board and I am happy to advise.