They first showed up in the mid '80s in the Chinese imports, most were shortened 38s, a few were the original length. Some had (1) unmodified followers, others had (2) notches cut towards the follower front and two "L" shaped pieces of metal welded in the mag box, the short "arm" or base of the "L" fitted through the notches in the follower when depressed holding the shorter 7.62 cartridges in towards the rear of the magazine. (3) same as two but notches cut towards the back of the follower holding the cartridges towards the mag box front and (4) back of follower was cut and an inverted "U" installed to keep cartridges towards the front of the chamber. I probably shot 50 or so and this last method seemed to work the best.
Rechambering apparently was done by (1) craftsmen, (2 ) idiots. I've seen cases ejected looking like they came from an SKS or AK and I've seen cases that looked like they had been made for a shotgun, shoulder completely blown out. They even shortened and modified some Type "I" to 7.62x39, I had SN D22 and heard of others. Many had the slots where the band retainer springs were originally installed filled with rectangular pieces of wood, others were left open. (Bands and springs had to be moved when the rifle was shortened) I have one that apparently was one of the last ones modified at whatever facility did the work, the front part of the barrel is still in the white. The late Roy Cooper saw one at a TN show with bring back papers and I saw one with a presentation plaque on the stock, to some officer from his men, I'd have liked to have had it but $500 was too much w/o documentation. (or with documentation!)
The Birmingham Pistol Parlor sold them for $25 + tax, three of us drove to the importer in Savannah, GA and were charged $17.50 each. "Those were the days my friend..."
As I remember a chamber insert was used to change caliber?
In Australia, several T38 Carbines (yes the real "Carbine") showed up with the 7,62 x39 Conversion
Only markings on metal work was the Chinese character for "Modified" stamped on barrel shank, in very small stamping ( about 4,75mm --3/16th high)
My carbine also has "Min Bing" (peoples Militia) Butt markings.
Magazine modified by machining back of elevator, and inserting a U shaped piece of metal in magazine well, to keep cartridge forward. meant to be used as a single loader, fed from magazine, to ensure proper extractor engagement on rim.
Barrel is original Japanese ( at least the major part of the shank is, up to the rear sights). Can't make out any "sleeving or "stubbing";
Movie newsreels of Gough Whitlam (Aust. Socialist Prime Minister 1972-75) visiting a Commune in 1974-5 shows Young Pioneers cheering him and Chinese Prime Minister Zhou En-lai, and carrying T38 Carbines (Most probably conversions). Photos of same visit also appeared in a English Language China weekly propaganda journal, "China Weekly Illustrated" (or similar name). photos are veery close up and detailed.
As to my example, I tried Military steel cased ammo, and 50% of the time, it failed to extract (not enough extractor grip); I ended up making "SEMIRIMMED" 7,62x39 from .303 cases (brass) by turning down rim to 6,5 Japanese dimensions (.475) and allowing a deep extractor groove, and forming and anmnealing the brass...shoots well and extracts regularly.
One could also make semirimmed ammo using .220 Swift cases without rim turning...the semi rim is the same as the 6,5 Japanese case.
The Chinese also converted the Longer T38 Rifle into a Militia "Short rifle"...quality of conversion depends on who actually did it. Almost all were meant for training and target practice, and blank fire...in a single round mode only. I have seen NONE in Australia (or China either) converted to use the full magazine capacity. At the same time, excess Vz26 LMGs were converted to use 7,62x39 with a AK47 30 round magazine (1960s-70s), as were some J.Inglis 7,92 Bren Guns (Lend Lease 1942-44).
Those imported into the U.S. most anyway would feed the 7.62 round with little problem, same with the modified T-I. I have a T-99, 11th series, with SKS front and rear sight and folding b'net still in original cal, also have a 38 with the SKS folding b'net, original ladder rear sight, rifle converted to 7.62. At some show during this past year a young man (anyone under 65) brought in a 99 (pretty sure it was a 99 and not a 38 'disremember') with the side-folding Russian/Chinese b'net.