A Monk's Unarmed Strike only counts as a natural weapon for the purposes of things that enhance or improve natural or manufactured weapons. You can treat it as a natural weapon for a Necklace of Natural Attacks or as a manufactured weapon for Greater Magic Weapon, but you can't say it's a natural weapon for how many attacks you get in a full attack.
if that's the case then.... Strength Devotion's slam attack would be at your full attack bonus... and my DnD group has been missusing monks UAS for years...
No and maybe.
Like I said above, Unarmed Strikes follow the rules for manufactured weapons (eg iterative attacks, etc). If you are attacking with both an Unarmed Strike and a Natural Weapon (such as a slam attack), then you'd follow the rules for combining weapons on page 100 of the Rules Compendium.
COMBINING WEAPONS
Some creatures combine attacks with natural and manufactured weapons when they make a full attack. When they do so, the manufactured weapon attack is considered the primary attack unless the creature’s description indicates otherwise and any natural weapons the creature also uses are considered secondary. These secondary attacks don’t interfere with the primary attack, but they take the usual penalty for being secondary attacks, even if the natural weapon used is normally the creature’s primary natural weapon.
Thus, you would make your full allotment of Unarmed Strikes (including bonus attacks from Flurry of Blows), then attack once with each natural weapon you have, treating all of them as secondary natural weapons (regardless of whether they're labeled primary or not). Since Flurry of Blows applies a penalty to all attacks made in that round, your natural weapon attacks would also receive that penalty even though they're not technically part of the flurry.
For further clarification:
If a creature with multiple natural attacks (such as the standard two claws and a bite array) takes levels of monk, how do flurry of blows and its natural attack progression interact?
You can’t use a natural weapon (claw, bite, or whatever) as part of a flurry of blows—only unarmed strikes and special monk weapons can be used in a flurry of blows. If a creature can use one of its natural weapons as a secondary attack in conjunction with manufactured weapon attacks, it can do the same with that natural weapon in conjunction with a flurry of blows. Any penalty assessed on attacks by the flurry of blows would also apply to the natural weapon attack. For example, a typical lizardfolk can attack with a club and its bite as part of a full attack. Normally, a creature would take a –5 penalty on an attack roll made with a secondary weapon, but since the lizardfolk has Multiattack, the penalty on the attack roll is reduced to –2 and adds only half the lizardfolk’s Strength bonus on the damage roll. If it were a 1st-level monk, it could make a flurry of blows (using unarmed strikes, not claw attacks), then add a bite attack as a secondary attack. Each unarmed strike would have a –2 penalty (from flurry of blows), and the bite attack would have a –4 penalty (–2 from flurry of blows and –2 from being a secondary weapon, reduced from –5 by Multiattack).
