I just noted that on page 45 the description of File Type 06 says; "APL continued file-5100 only" This more or less confirms what I remembered: The first was a 5100 then latter there was a 5110 and finally a 5120. Incidentally, and this is from memory, when IBM released the first 8088 based PC it was officially the IBM 5150.

I suspect that the comment you got about the photo being a 5110 is correct. He seems positive about it. And I think if the 5110 had disk instead of the tape it was in an external box. The photo appears to have the tape slot simply covered, which was not an option on the 5100.

APL was an extra cost option BASIC was standard. (IBM did some very dumb things, or APL would have been the major language today instead of C, obviously my opinion; but it is correct!) The photo unit does not have APL because: above right side of the keyboard there is room for 4 rocker switches (It looks like 5 but the left position was never used.) The left switch (which is on the photo) was the "REVERSE DISPLAY" this determines whether the display is light characters on a dark background or the opposite. The next switch is the BASIC/APL switch "Only dual-language machines have this switch." The third switch is a RESTART, and the fourth is DISPLAY REGISTERS/NORMAL for use by service representative's.

"The 5120 uses the 5110 Model 3 computer." It has two built-in diskette drives and a larger screen.

The IBM 5100 actually emulated the IBM 360 and ran the 360/APL interpreter. IBM was paranoid and afraid somebody might run 360 programs on the 5100. Good engineering, but stupid marketing. But then what else is new? As far as I know the last time IBM successfully marketed any software it was FORTRAN, for the 704. But dam, their machines are reliable workhorses. (With parity, at least initially!)