Samsung open source releases for some legacy Broadcom-based devices (mainly Rhea platform) and a few Qualcomm ones too, because why not? Just clone/download the branch you need!

• GT-S6102B - Galaxy Y Duos (torino)

• GT-B5330 - Chat (zanin)
• GT-B5330 - Chat (zanin) - Europe variant (MEA region)
• GT-B5330L - Chat (zanin) - Latin America variant
• GT-S5301L - Pocket Plus (coriplus)
• GT-S5310L - Pocket Neo (corsica)
• GT-S6010L - Music (ivory)
• GT-S6790L - Fame Lite (nevisvess)
• GT-S6810L - Fame (nevis)

• GT-S6293T - Young Duos TV (royvedtv)

• SGH-S730G & SGH-S730M - Discover (amazing)

• SM-G900F - S5 (klte)

These describe which version of Android the sources are designed for:

• GB = Gingerbread (2.3.x)
• ICS = Ice Cream Sandwich (4.0.x)
• JB = Jelly Bean (4.1-4.3)
• MM = Marshmallow (6.0)

These appear to be entirely useless for custom ROM development as they're simply generic AOSP trees with no device info filled in. Probably were thrown in there to look good to open source community.

Just enter your device's model number at Samsung Mobile Open Source. If you can't find the sources for your device, create an inquiry at Inquiry Form asking for your device's sources since they are good people and usually answer within a day.
You can create an issue here if you're too shy to ask them :)

Thanks to @Hacker-Anirudh for adding the GT-B5330 europe variants and fixing a few things in README!