I agree with XLH59 that his frame has a 57R neck based on the XL style top motor mount, no front downtube "doublers" and the reinforced neck design.
My question is what year and models used the 57R neck casting? The photo of the 57XLR-TT I believe is using the -52 neck casting. Should it be the -57R casting or the -52 casting? Were there only two racing frame part numbers, the 52R and the 58R I've seen mentioned in this thread? Did the 57R neck first show up with the 58R frame mentioned? If the 57XLR pictured is using the -52 neck because it is an early mock-up I understand. If you look closely at the photo of the mock-up 52K you can see that its early K frame is lacking the welded on strut supports.
I believe the 52K, 52KR, KRTT, KRM and 57XLR-TT are all displaying the -52 neck casting. I would guess that the 56KRTT photo shows a -52A neck casting. An easy way to ID the -52 casting is its lack of neck reinforcing. All subsequent neck castings I am aware of are reinforced like the -52A shown.
I'm thinking the 56KRM shown is using the -52A casting based on no doublers and being reinforced.
I admittedly know little about racing frames and bikes so any corrections for my assumptions or additional racing frame information would be much appreciated.
JerryR