Hofs01, heres pics of my 158 topper. It came as a 20ga, and I sent it back to the factory and had 22 Hornet and 30-30 bbls added. Its shown with the Hornet bbl on, and with the 30-30 bbl and a new 20ga bbl that fits it perfectly
I think you may possibly be mistaken or miss informed, concerning your barrels...
I am not saying the factory didn't fit barrels to your receiver for you. They just no longer will. My questions are about the barrels and there age.
I would like to point out the stud, under the barrel, where the for-end will attach. We can see two of them in your second picture. The top one with the scope is an old style barrel with the old style stud, second generation if you will. The lower barrel, with open sites, (20ga I assume) is a newer style stud. Also the side of your receiver looks newer than a M157/8 series. (But I cannot see it that well, so I could be mistaken) The 157/8's said 157 or 158 right in there sides and most also had low profile hammers. I do not believe the new style for-end stud was offered on the 157 / 158 receivers.
Here is some pics to clarify. This is a newer style fore-end stud like your shotgun barrel appears to have:
This is a old style, your scoped barrel (not mounted to receiver)in the second pic, appears to have:
For reference, ere is a 158 receiver:
This is a early 158 receiver with a STD hammer. Notice lack of SN in normal position:
Did you have barrels that you sent to the factory? That could account for the differences... Again, to my knowledge, the factory will not fit a barrel you send in any-longer.
CW