bally made a lot of different versions of the 742A, with one of the differences being the reel tape symbols.
if you game only has 7's, bars, maybe melons and blanks, it'd be one of the "jackpot only" style games that pays less frequently (statistically), but pays more for any win.
the 742a-m186a is probably the serial number ... the bottom number on the plate below the handle. The top number on the plate is the model number, and sometimes it matches the game :-)
if you're having trouble posting pics due to size, you can email highest possible resolution images to slotpics@cdyn.com and I'll downsize and post them for ya. Entire front of the game, serial number plate, codes stamped into the slotted metal reel index discs and the m-645-xxx number on payout counter disc under the hopper helps identify a game. There may also be a model number written in black sharpie on various units or on the reel wiper boards.
any 742A schematic will be mostly correct except the reel wiper wiring and the payout disc. That is specific to the type of machine and rarely may be unique to a specific model (e.g. a 742A-130A "tropicana special" model made for the tropicana casino had unique reel wiper wiring afaik).