This story is a sequel for Barks' "The Mysterious Stoneray" (W US 8), with references to "Terror of the Beagle Boys" (WDC 134), and "The Big Bin on Killmotor Hill" (WDC 135) .
First, this panel is from "The Big Bin on Killmotor Hill" :
This scene of the cannon have been used several times in Barks' stories such as "The Pixilated Parrot" (OS 282), and "Terror of the Beagle Boys" (In 1973,Carl Barks also made an oil painting entitled "Who's Out There?" with this cannon) :
This one is from "Only a Poor Old Man" (OS 386) :
Then, those two panels are from "The Mysterious Stoneray". Just look at the cabbage professor's posture :
Finally, in this panel, Don Rosa refers to Needle Hill and Snag Hill from "Searching for a Successor" (WDC 187) and to the Old Mountain Demon Tooth, from "The Money Stairs" (WDC 157) :
A panel from "The Pixilated Parrot" is also used in "Incident
at McDuck Tower" (D
90345).
Panels from "The Big Bin on Killmotor Hill" are used in "The
Guardians Of The Lost Library" (D
92380), and "His Majesty, McDuck" (AR
145).
Panels from "Only a Poor Old Man" are used in "Last Sled
to Dawson" (AR
113), "The King Of The Copper Hill" (D
92083), "The Billionaire Of Dismal Downs" (D
93121), "The Empire-Builder from Calisota" (D
93288), and "The Richest Duck in the World" (D
93488).