Hello SOC gents, I am trying to find out if the washer/ rivets attached to the bottom right and left outsides of the body indicate whether this is a Pan or Tube frame? 
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That little round cover (which was also found in the original Porsche cars) is a hole used when you want to change the rear ride height and/or remove the rear torsion bar spring plate for service (needed almost never once initially set up).
It does not indicate whether the car is a pan or tube based chassis.
Other IM owners on here can tell you when IM started building tube frame cars.
From a 2010 MotorTrend article -
As with most of the firms still replicating the 356, Reisner's early kits were designed to ride on shortened VW Beetle floorpans, of which there was an abundant supply in the '70s. But in 1985, the company engineered a greatly improved square-tube chassis to replace the flexy VW pan while still accommodating the VW suspension and running gear. A year later, this new chassis was enhanced to accept Porsche 911 suspension and running gear, seriously amping up the performance.
Thanks, gentlemen!
9 out of 10 times, if the shifter tunnel is round, it's a pan. If it's square, it's a tube frame. There are only a few owners that have gone through the trouble of squaring up the round tunnel for looks.
-=theron
Were the IM tube frames like the picture above for straight axle VW only ?
I do not see any mounts for the stock VW IRS trailing arms/
Thanks for the info
That's not an iM frame. That's a Brazilian Beck, unless I'm badly mistaken. They were swing only cars, the IMs were and the new Becks are IRS.
The frame in the picture is definitely an early Beck frame, which is setup for swing-axle only, but I think it was posted as a generic example of a tube frame. The IM tube frame used square tubing and was able to handle ISR in back and (optionally) 911 suspension in front.
