Hi Rodger,
Here's a quick run through TIU history...
Rev. G - This was the original, released in April, 2002. Unlike all later TIU's, the Rev G's had the black terminals on all four channels tied together. They are also the only TIU revision without internal blade fuses and were vulnerable to shorting out with older transformers with slow acting breakers. The G's can be identified on the outside by the lack of a revision sticker on the underside. The G's also have a "ProtoSound 2" logo painted on the top of the case.
Rev. H1/H1A/H1B/H2/H2A - The "H" revisions added the blade fuses for auxiliary circuit protection and isolated the black terminals on each channel. The G and H revisions all used 5 volts chip sets. The various alpha numeric iterations within the "H" revisions represented minor hardware changes on the circuit board. All "H" revisions came with an identifying sticker centered on the underside of the case. The H1's were painted with the "ProtoSound 2" logo, but from the H1A on MTH switches to the DCS oval logo on the top of the case.
Rev. I/I1/I1A/I3/I3A/I3B - The "I" revisions moved from a 5 volt to a 3 volt chip set. The various alpha numeric iterations within the "I" revisions represented minor hardware changes on the circuit board. All "I" revisions came with an identifying sticker centered on the underside of the case. MTH used a sans serif font on the revision stickers, which has often lead to confusion. Many people mistake the "I" for a "1."
Rev. L - So far there has only been a single variation of the Rev. L. The L uses a 3volt FPGA chip set with improved signal recognition, which effectively improves track signal quality. The "L" also added a USB-B port, which can be used in lieu of the RS-232 port to connect the TIU to a PC for updating firmware and transferring files. You can identify the Rev. L by the sticker and the USB-B port.