If I had such a plane show up in my shop for inspection, this is what I would do.
First I would look under TC 1A2.
and would find this:
Serial Numbers Eligible 18-3771; 18-3781 through 18-9015; 18-7309016 through 18-8309025; and 1809001 through
1809113.
since its a 59 model, it would fall in this number sequence.
Then I would go here:
(1) Airframe major alterations. Alterations of the following parts and alterations of the following types, when not listed in the aircraft specifications issued by the FAA , are airframe major alterations:
So no 337 is needed as they are listed in the TCDS
Then I would go here:
Sec. 91.417
Maintenance records.
(a) Except for work performed in accordance with Secs. 91.411 and 91.413, each registered owner or operator shall keep the following records for the periods specified in paragraph (b) of this section:
(1) Records of the maintenance, preventive maintenance, and alteration and records of the 100-hour, annual, progressive, and other required or approved inspections, as appropriate, for each aircraft (including the airframe) and each engine, propeller, rotor, and appliance of an aircraft. The records must include--
(b) The owner or operator shall retain the following records for the periods prescribed:
(1) The records specified in paragraph (a)(1) of this section shall be retained until the work is repeated or superseded by other work or for 1 year after the work is performed.
since the modifications were done according to the TC they would be considered minor alterations and the records only need to be kept for a year.
We all know what happens when a Supercub goes to Canada. Those knuckleheads throw the US logbooks away. Or they could have got lost or burned up , or stolen, but they aren't required.
So as far as I'm concerned, up to this point everything is OK.
Now we get into a couple gray areas.
1. The Data Plate.
Does it have to say PA-18-150 ? I cited earlier a case where they weren't changed and this was done by Piper, so we have precedence. Also, If you have a AR-7 certified cub and have a standard AW cert, you are operating under a completely different TCDS than the AC was certified under and still have the AR-7 data plate. So why would this one be illegal. Also, suppose it had a -150 stamped on it by persons unknown, does this make it an illegal alteration of the Data Plate? Why?
2. The Airworthiness Certificate. Does it have to say -150? if so why ?
I would document all the above info and use it to justify sending the airplane on its merry way as long as everything was airworthy.