[size=medium]Elton,
The earlier Classic series, in the more squareish control housing and powered by 3-9V batteries, operate at 6.59 kHz and share the same search coils EXCEPT the Classic III PLUS. The Classic III Plus also operates at 6.59 kHz, but the front-end electronics make it function with the Blue max series of coils, like the 950 it was supplied with.
Then there are the Classic SL series. The ones using the same slim-line housing as the XLT and other newer models. The Classic I and Classic II operate at 6.59 kHz, but use the old-style circuitry and do NOT use the newer Blue Max series of coils. Matter of fact, both the old-style and 'SL' style Classic I's have a hard-wired 8" coil. Only the Classic II SL interchanges with the earlier type coils.
The Classic III SL, Classic ID, Classic IDX and IDX Pro all operate at 6.59 kHz and share the same Blue Max series of coils (and others that do not say "Blue max" such as the Royal 800, etc.) as the XLT, Spectrum, Eagle II, 5900 & 6000/Di Pro SL, 6000 Pro XL and renamed XL Pro, QXT Pro/Quantum XT and Quantum II and a few others.
The Classic 4 and Classic 5-ID produced for Kellyco operate at 8.1 kHz and do not share coils with any other White's, except for maybe a few special package things they worked up for Costco, etc.
Monte[/size]