Loose RS232 on TIU

Couple of things going on here.

#1 The yellow you see was likely Kapton tape (a very high temperature tape) used during assembly and wave soldering of the typical PCB to prevent the solder from sticking and or shorting connectors and places the solder should not flow into or coat.

https://www.youtube.com/watch?v=lvh7G8f0jOU

It would seem to me from that picture you are missing the security screws/standoffs? Those at one point in time were common parts- something Radioshack or other electronics stores would have.

Keyword search at MTH parts "TIU" https://www.mthpartsandsales.com/shop/search/results

"I was considering installing plastic dowel or screw but am concerned about creating a short. Is it safe to do this or should I consider a different option?"

I'm slightly confused by this, and unsure if you meant somehow replacing the rivets holding the black plastic connector outer shell (not the contacts) ?

These are replacement snap in style connectors if the connector is really damaged. Snap meaning you unsolder the  old connector, remove the existing rivets, insert this style snap in and the clips hold it before soldering, and then solder the connections. Unsure if the rivet holes have through plated copper to solder to as my rivets completely block any view of the PCB hole. Found on the A site "PC Accessories - Connectors Pro DB9 Female Right Angle PCB Mount Connector, 90 Degree D-Sub"

If the actual rivets are loose, you could either consider pressing them to tighten them or carefully removing them and using small screws to secure the RS232 connector.

Mating USB to serial ideally would have the matching security screws

But let's say yours doesn't or is the opposite?

I bought these for use with my DZ2500 wire line driver module in my Legacy setup but just as useful here.

"Five Pack of YCS Basics 1 Foot DB9 9 Pin Serial / RS232 Male/Female Extension Cables"

Also a search for "DB SUB standoffs" on the A site came up with both standoffs and security screw examples.

I also agree after looking at both sides of the TIU PCB, they used metal rivets, so replacing a loose rivet would be fine, it's not providing electrical contact nor should short out. Again,  if there was Kapton tape, it likely was part of the soldering process and removed, it wasn't for permanent electrical isolation.

However, if that's the problem and loose, bigger fear would be cracked solder joints to the actual 9 pins of the connector or worse, cracked thin PCB traces.

Picture take from this topic by @Adrian! https://ogrforum.com/topic/79142442692986004

Ignore the yellow circle in this photo- the connector and solder joints are on the left. the 2 big silver circles are the rivet heads.

Another option for loose rivet heads if you are reluctant to try to replace them is my favorite adhesive, JB-Weld.  I have repaired the power jacks on a bunch of the MTH WiFi units by reattaching them and then using JB-Weld to insure they don't get loose again.

Thanks all.  I replaced the rivets with small screws & used locktite on the nuts. I also reflowed the 9 pins of the connector.

Great point on the standoffs! I totally forgot about these and by not using them was the reason the RS232 got loose.  Especially with the weight of the Y connector required for WIU/Legacy split. I  also plan to zip tie the Y connector to something to avoid the side forces on the RS232.