105 vs 115 is the first three digits of the VIN. Early examples (I think all round tails and perhaps very early kamm tails?) are 105, later 115. Pretty sure your '74 will be a 115. Anyhow, commonly referred to as the 105/115 series. Most parts are interchangeable.
As far as a shop manual goes, you can order one all printed out from Centerline, IAP, etc. Or you can get a big collection tailored for your car including service manual, owners manual, part manuals, tech bulletins, etc. etc. etc. free in electronic form from the Alfa Romeo Owners Club - ask for the "all-in-one":