Ok, sorry if this seems wordy or like I am teaching a class but it really seemed the easiest way to do this, but here you go....
Naturally we all knew that when Tom Baker took over the role he had used a copy of the Pertwee ankh key in promotional photos as the real one was gifted to Pertwee himself when he left the role. Promotional pictures of course are great as often times they can give us a better look at the props than the quality of footage and angles of the episodes but they can also be misleading, as is partly the case here for Tom. Based on these photos it was assumed that the only ankh key Tom used in the show was the same and therefore it was believed that the Pertwee copy was the first and only ankh key Tom had. That said, a few of us believed that there was another key as screen grabs and the promo photos didn’t always add up. Well after an exhaustive search, Straker and I feel we have finally gathered enough evidence to prove this theory and when we did we even shocked ourselves. What we found was that the mystery key – The Baker Key -, that we felt so strongly had existed, appeared in the show long before even we thought it did; Tom’s first episode as it turned out, Season 12’s “Robot”. The Baker key is pretty much the same style as the Pertwee key however the construction is simpler and some of finer points are very visibly different. To name a few:
1) Diamond top
2) One layer construction with no lip on the back
3) Pointed bottom
4) Larger constellation design
The Pertwee copy was still used in the Baker era however, not just in promotional photos. It can clearly be seen in “Genesis of the Daleks”. This is what I meant before by the promotional photos sometimes being misleading, yes Tom did use the Pertwee key copy onscreen but it wasn’t his first key as was previously thought, it was actually his second. In fact it seems that the Pertwee copy may have only been used in “Genesis of the Daleks” and “Terror of the Zygons” - the last story filmed of Season 12 and the first story filmed of Season 13. Admittedly the Pertwee copy could have also been used in the two stories filmed prior to Genesis - “The Ark in Space” and “Revenge of the Cybermen” - however the reference pictures are just too hard to tell any real definition either way in Ark and the key remains in his cardigan pocket during Revenge. It would then be fair to say that at most the Pertwee copy could have been used in at least two to four stories while the Baker key lasted much longer having been used before and after the Perwee copy. The last known use of the ankh style key in the Baker era appears to be in “The Sun Makers” in Season 15 where, again, the due to film quality it’s too difficult to pinpoint which version of the ankh key it really was.
While doing the research to prove the existence of the Baker key an interesting fact was also discovered on the Pertwee copy as well. The uneven crossbars of the Pertwee key are a well-known signature of the piece and it was believed that the Pertwee copy retained this feature. However looking at the screen caps from “Genesis of the Daleks" it is apparent that the props department had corrected the mold to have symmetrical crossbars. Which is interesting because when Mooncrest Models originally made his Third Doctor key replica a few of us had asked him if he could release an idealized “Tom Baker version”, which in effect was a simple alteration to his Pertwee key design making the crossbars symmetrical and Gary very kindly obliged. At the time we did this because we wanted a key that would be unique to Tom Baker in our collections but as it turns out when we asked Mooncrest to make that idealized key we in effect were actually making another screen accurate key without knowing it!
So there you go, visual proof that a Baker specific key does/did in fact exist and some updated facts about the Pertwee copy key. And not, some pics collected from many episodes to show all these points, which as they say are worth a thousand words.
EDIT (to Add Adams comments below in one place):
In the Baker Key pics below, specifically the key in the lock of the "The Android Invasion" group, you can see the key has the same layer thickness to the cross arms as it does the head the chain passes through, which shows very well that it is a single layer construction.
The Baker Key:
The Pertwee Copy:
First time poster to the RPF here. I have browsed around a few times after hearing about it from Adam Savage. But today I made my first actual replica prop thats not made of paper. I made a perception filter key. I just grabbed a few blank keys with a round top. I hadn't seen this thread until after I had started making the prop. I don't know whats worse, the fact that I had everything but the blank key in my bedroom or the fact I am making props now (not actually bad just nerdy). But I used a dremel to key the key to look like this pattern (http://hotgeekz.com/wp-content/uploads/2014/05/clip_image014.jpg) then used this still posted by Terrasolo (http://i175.photobucket.com/albums/w128/terrasolo/3x13-The-Last-of-the-Time-Lords-1_zps8fcb166c.png) to super glue all the components on. I took a simple 8 pin IC (a 555 timer works) and cut the legs off then sanded the botom to be rough for the glue to hold. The cut up a sim card for the gold contact pad, found a resistor and then some wires from my old computers power/led for the red and black wires. I didn't solder anything since I didn't see the previous posted replica done by straker until afterward. But I am still happy with this for it being my first prop. I have plans to visit a local locksmith and see if I can find a more accurate yale or ERA key. Here is my finished product. 
OH yes and I figured this is worth sharing.
