on mello's (many) motorcycles and guns:
mello has two motorcycles in the manga: the one he uses to tail misa, and the one he uses to kidnap takada.
(images edited by me, #solomellomangapanels)
the first one is unidentified. most likely because this model doesn't actually exist. the engine looks like it's either an ironhead or shovelhead, but the fairing and front headlight are definitely not harley. it's got an early 2000s, rectangular silhouette that's reminiscent of japanese motorcycle models. but as far as i can tell, we never see the rest of this particular bike.
maybe obata spotted a custom harley in a motorcycle magazine as custom motorcycle culture was big in japan at the time, or perhaps he just mixed and matched elements from motorcycles he liked. however, most of the vehicles and weapons in the manga have real-life counterparts, and mello's second bike is clearly based on the BMW K1200LT. so he very well may have referenced a custom-modified bike, likely from japanese motorcycle magazines from the early 2000s like 'custom burning'. i tried looking through old magazines (2001-2004, since the manga ended in 2006 but apparently obata finished his work in 2004), but most of them don't have digital versions, and can only be bought secondhand.
the K1200LT gets changed into a Honda NR 750 in the anime.
@yagami-raito-kun already has an amazing and detailed post on mello's gun in the manga. a quick summary + additional info:
in the manga, mello's gun is a Beretta 92FS INOX. the design was inspired by benvolio's gun from the brazilian movie "romeo and juliet". however, benvolio's gun was a Taurus PT99. obata may have confused the two models while looking for reference pictures, since they appear very similar.
the Taurus PT99 is essentially a direct descendant of the Beretta 92. in 1974, beretta built a factory in são paulo, brazil, to produce pistols for the brazilian military. when the contract ended in 1980, beretta sold the entire factory, machinery, and tooling to taurus. the Taurus PT99 is simply taurus's adjustable-sight version of their PT92 (which is their original copy of the Beretta 92).
the two models can be distinguished by where the safety lever is located: the Taurus PT92 usually has a frame-mounted safety, while the Beretta 92FS has a slide-mounted one.
(in order: the 92FS INOX, PT99, benvolio's gun, mello's gun)
in the anime, however, mello's gun was changed to a Beretta 92FS "Sword Cutlass/Scimitar", a nod to black lagoon. this gun is a custom version of the Beretta 92F with a 5.9-inch barrel and slide setup, making it 1 inch longer than any standard Beretta 92 model.
