Abstract : The groups of the title were first characterized by Janko in terms of the centralizer of a central involution. If there are two classes of involutions, the group is the Hall-Janko group of order 604,800 = (2 to the 7th power) x (3 cubed) x (5 squared) x 7. It was first constructed by M. Hall, Jr. and we denote it by J sub 2. Otherwise there is only one class of involutions and the group is of order 50,232,960 = (2 to the 7th power) x (3 to the 5th power) x 5 x 17 x 19. This group was first constructed by G. Higman and J. McKay. We denote it by J sub 3. The main result is that the multiplier of J sub 2 has order 2 and that of J sub 3 has order 3. A consequence is that J sub 3 has a projective (complex) representation of degree 18. (Author)