Difference between T42 and M24 22 long rifle cartridges?

I am trying to figure out how to tell the difference between T42 and M24 22 long rifle cartridges. HWS 2 p.224 says T42 cases had one knurled case cannelure .20 from the mouth and M24 cartridge eliminated the cannelure, but adds (not confirmed on rounds examined). T42 bullets weighed 42 gr. and M24 bullets were shortened and weigh 40.5 gr. FIG. 207 gives OAL of M24 as .995" - .02"
Does anyone have one without a cannelure or have 2 to compare for OAL or weight?
The 2 that I have both have a cannelure , measure .985" OAL and appear to be identical.Any help will be appreciated.

Have these two with the larger U measuring .989" and weighing 52.9 grains
The smaller U measures .9875" and weighs 52.1 grains
Both have knurled cannelures.
Ave two others ball rnds with very tiny variations & all have the knurl, even a blackened case dummy.
Also a box (holding the larger HS.ed var. & an empty foil-backed paper wrapper.

I have the T-24 boxes with Lot numbers 5000 and 5000R and M24 boxes with Lot numbers 1-1, 5000, 5001, 5002, 5003, and 5005. The two opened T-24 boxes and all the M24 all have cannelures. Placing the two cartridges side by side I can see no difference.


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Pete and Rich
The two I have weigh 51.8 and both have the small U/hs
The weight difference is so small and so is the OAL maybe there is no way to tell the two apart unless they come out of a sealed box or someone finds one with no cannelure.
Thanks for looking

I cannot say for certain that this is the case for these cartridges, but in general US ordnance goes through a test/development and evaluation phase. Prior to about 1957(? memory) during this phase different test items were given a T-number. After that year it changed to XM (experimental).
For larger ordnance the latest versions of the test items were the exact same item as the adopted piece, simply re-designated. The 155mm T77 for example became the M121 once adopted. In many cases they used up the developmental projectiles, keeping the stamped markings of T77, but stenciling them as M121. This later (much later) caused confusion for treaty compliance teams when they found rounds stenciled as one thing, but stamped as another. The .22 M24 could possibly be a similar situation.

I think for now I’ll just label these T42 - M24
You might be correct and this was more of a name change.