Given the information you've supplied, we almost have enough to generate the original HIN.
See the page here at
https://www.pearsonyachts.org/hull-identification-number.htmlfor details.
Since the boat was built before 1984, it uses the older form of the HIN:
Mfg + Model Code + hull num + "M" + year + month
As you stated, Pearson's Mfg code is "PEA".
The only 26' model produced in 1973 is model code "46"
Hull number is "558"
Year is "73"
Month is ? ("A" through "L")
Thus, HIN = PEA46558M73?
This gets you close, but close isn't good enough.
Further research shows that over 1700 P26's were built. Hull numbers in the 1976 timeframe were in the 1200 to 1369 range. Since this exceeds the three digits allocated for hull number, they get truncated, often confusing things. Since yours was built in model year '73, it's unlikely that the hull number exceeds 1000.
The best way I can think of to determine the final "month" character is to locate the HIN for either hull # 557 or 559. These are almost certain to have been built in the same month as yours (hull #558).
With this in mind, I poured through Dan Pfeifer's old P26 archives, and found the following info:
1973 -- Peg & Bill Gray ( # 558, Esprit ) [ irish541@jersey.net ] Upper Chesapeake Bay MD
1973 -- Gary Mckenzie ( # 559, Diamond LiL ) [ Gary478@aol.com ] West Palm Beach, FL
Now I'm guessing you already have the first line, but the second line may give you another lead to follow.
Good Luck!