I want to build an ISB kit for an officer/ agent but I am not able to find the specific and 501st approved rank bars for an ISB Lieutenant Colonel.
In Andor they are using Major and Colonel, but this one is missing?!
Are you saying there's no Lieutenant Colonel in the show? If so, that would presumably be why there's no 501st approved rank bar for an ISB Lt. Col. I'm speculating here, but my understanding is they only approve things that have appeared in official publications. We can conjecture that there are Lt. Colonels somewhere off screen, but we don't know that. Heck, perhaps the ISB doesn't have the rank! A Lt. Col is an "assistant Colonel," an officer there to help the commander of a Regiment keep control of a couple thousand soldiers. Maybe the ISB jumps from "Majors are in charge of major detachments" to "Col. Wularan is in charge of everything."
For a real-world parallel, in Canada the Royal Canadian Mounted Police's rank insignia are based on the British Army's, but skip over Second Lieutenant, Lieutenant, and Captain, and go straight to using a Major's Crown for a Superintendent. They then count up as normal until they get to using the Brigadier's insigna for an Assistant Commissioner, but then jump over the Major General's insignia to the Lieutenant General's for the Deputy Commissioner.
So, perhaps, just as there is no RCMP uniform with either a Captain's rank insignia nor a Major General's, perhaps there is no ISB uniform with a Lieutenant Colonel's rank bar. The ISB isn't actually a military force, after all. (Actually, now I'm really sad that the Major is addressed as such on screen, because it would have been awesome if the ISB used British Imperial police titles instead...)
Or are you saying there is a Lt Col in the show but you can't find the 501st's official specification for the rank bar? If so, I'll have to leave that for people with more ties to the 501st to anser.