Did McIntosh ever publish the OEM and gel part number they used? Couldn't find anything online. Seems like a mystery. McIntosh used a blue-tinted theater gel in front of the magic eye tube and both sides of the dial glass on tuner or tuner/preamp models like the MX110, MR67, MR71 etc. Run a search and you'll find part numbers and gel colors used by AK members as a replacement are all over the map. For example, I've seen different posts say use "Lee Steel Blue #117" or "Rosco Nile Blue #70" or " Times Square Lighting Parts Express #244-141 (118 bright blue)." I decided to see how the Lee #117 compares with the original gel McIntosh used in the dial glass location. See image below. In comparing the two side by side, the original 1960s McIntosh gel was significantly thicker and seemed more robust than LEE 117 -- as if the old MAC gel was designed for high temperature applications. The LEE 117, in contrast, was quite thin and seemed less suitable to being in front of a hot tube or bulb for a long period of time. The colors were close but not precise. The original gel McIntosh used has a slight bit more green and the added thickness makes it a bit more opaque. I think there is probably another gel model # that represents what McIntosh originally specified. The LEE #117 doesn't come in a high temperature version. But LEE makes another gel called #172 Lagoon Blue that has more green and is offered in a high temperature variant and might be a more robust and authentic option than the #117. I found an old post talking about Rosco # R70 Nile Blue. Not sure if the Rosco is thicker like the original MAC gel. The LEE equivalent to a Rosco R70 is apparently the LEE #140 "Summer Blue." The Rosco promo verbiage says "Rosco's premier range of filters are manufactured in a unique technology to insure the longest possible life under hot theatrical lights. Three discrete layers are combined in a tri-extrusion process. By sealing the colored layer between two microscopically thin layers of clear film, dye migration is minimized and effective life is extended." So perhaps the Rosco is thicker and more robust than the LEE 117? The Times Square Lighting 118 sold by Parts Express appears to use the same color numbering scheme as LEE. So in theory it should be the same color as LEE #118. If we take all three gels folks are using and equate them to LEE equivalents, below is a swatch image for comparison from the LEE web site. LEE part numbers below: 118 - Light Blue: Times Square (Parts Express). Lee offers equivalent in a high temperature. 117 - Steel Blue: Lee 140 - Summer Blue: Rosco R70 172 - Lagoon Blue -- Haven't seen this one used, but has a bit more green than 117 and is offered in high temperature. Wild Card: Rosco 363 -- perhaps a thicker more correct equivalent to LEE 117 So at this point, LEE #117 appears to be incorrect as a robust, color accurate replacement (unless a thicker ROSCO 363 equivalent was used), leaving 118, 140 and 172 as potential LEE equivalent gel options of what MAC originally used.