I remember a similar question has been asked a few months before by others. But I couldn't find the post.
Previous question is simpler than your one. It only has a "A-24" format.
Here attached 2 formulas which can be used in ALTEXT. (for TEXT objects only)
Instead of adding text in "PT.PT1.A:24-48" format directly, my solution is adding simple numbers without prefix in a sequence from 1 with some autonumbering tool first, such as INNB, then convert these numbers into A:24-48 format (with the first formula), then add Prefix such as "PT.PT1." to all these texts (also with ALTEXT)
The benefit of this workflow is, in case of error and you need add or delete a number in the sequence, you can easily convert them back to the original numbers (with the second formula), then add or minus numbers in a certain number range (also with ALTEXT), and then convert these numbers back to "A:24-48" format.
Step 1.1, Add numbers.

Step 1.2, Use ALTEXT and load Formula 1.

Step 1.3, Convert numbers to "A:24-48" format.

Step 1.4, Add Prefix "PT.PT1."

Step 1.5, the result:

Now, if there is an error in the sequence, for example, need add 1 to all numbers after A:24-48, ie. A:24-48 becomes B:1-1, B:1-1 becomes B:1-2, etc. We can use the second formula to convert texts into original numbers first, then use steps above to convert numbers into "A-24-48" format again.
Step 2.1, remove all prefix with FIND command

Step 2.2, the result of Step 2.1:

Step 2.3, Use ALTEXT and load Formula 2 to convert these texts into original numbers.

Step 2.4, Use ALTEXT to add 1 to all numbers ≧ 1152.

Step 2.5, the result of Step 2.4:

Then repeat Step 1.2 to Step 1.5 to convert the numbers into "PT.PT1.A:24-48" format.

Attachments:
Formula 1, Convert numbers into "A:24-48" format.
;;; This is an example formula for ALTEXT.vlx
;;; Convert selected numbers into mixed alphbat / numbers such as "A-1-1", last digit in a term of 48, first digit in a term of 24
;;; Revise corresponsive variables to suit
(defun ALTEXT_FORMULA (/ number TERM1 TERM2 CONNECTOR1 CONNECTOR2 PartA PartB PartC)
(setq TERM1 48 ;Changes it if the circulation is not 48
TERM2 24 ;Changes it if the circulation is not 24
CONNECTOR1 "-" ;Add space in front or after the dash as you need, or change it to any other character you need put in between the letter and number
CONNECTOR2 ":" ;
)
(setq number (atoi ALTEXT_STR_ORG))
(setq PartA (fix (rem number TERM1))
PartB (1+ (fix (rem (/ (1- number) TERM1) TERM2)))
PartC (chr (+ (fix (/ (1- number) (* TERM1 TERM2))) 65))
)
(strcat PartC CONNECTOR2 (itoa PartB) CONNECTOR1 (itoa (if (= PartA 0) TERM1 PartA)))
)
Formula 2, Convert "A:24-48" format texts back into numbers:
;;; This is an example formula for ALTEXT.vlx
;;; You can use AutoCAD command FIND to remove prefix and suffix first.
;;; It reverts mixed alphbat / numbers such as "A-1-1" back to its original number.
;;; Revise corresponsive variables TERM & CONNECTOR to suit.
(defun ALTEXT_FORMULA (/ TERM1 TERM2 CONNECTOR1 CONNECTOR2 PartA PartB PartC t1 p1 p2 L1)
(setq TERM1 48 ;Changes it if the circulation is not 48
TERM2 24 ;Changes it if the circulation is not 24
CONNECTOR1 "-" ;Add space in front or after the dash as you need, or change it to any other character you need put in between the letter and number
CONNECTOR2 ":" ;
)
(setq t1 (substr ALTEXT_STR_ORG (1+ (strlen PREFIX))))
(setq p2 (vl-string-search CONNECTOR2 t1))
(setq p1 (vl-string-search CONNECTOR1 t1 (+ p2 (strlen CONNECTOR2))))
(setq PartA (substr t1 (+ p1 (strlen CONNECTOR1) 1))
PartB (substr t1 (setq L1 (+ P2 (strlen CONNECTOR2) 1)) (1+ (- P1 L1)))
PartC (substr t1 1 1)
)
(if (and PartA PartB PartC)
(itoa (+ (* (- (ascii PartC) 65) TERM1 TERM2) (* (1- (atoi PartB)) TERM1) (atoi PartA))))
)